Download Website Lathyrus Species

Top left, Lathyrus aureus; centre left, Lathyrus sativus; bottom left, Lathyrus niger;
Centre right, Lathyrus rotundifolius; bottom right, Lathyrus aphaca “Modestus” (bright yellow).
“Affinis” (pale yellow). Photographs by David Archer.
Lathyrus Species
by D. F. ARCHER
Fordham, Ely, Cambs.
The difference between Vetches (genus Vicia) and ornamental peas (genus Lathyrus)
is not always obvious, but usually it can be said that the vetch flower is smaller, with
a smaller standard, the plant ends to have more leaflets, and the stem is not winged.
For some species, however, this is not a very workable definition and the botanist
adds another difference: in the peas the style of the flower is erect and hairy on one
side, but in the Vetches the style has a hairy tuft at the top and sometimes along one
edge. In the illustrations below, Fig. 1 is a Vicia style and Figs. 2, 3 and 4 are
Lathyrus styles:-
The Culinary peas (genus Pisum) differ from the ornamental peas in that they have
round stems and, more importantly, the wing petals of the flower are joined to the
keel.
The ornamental peas make up a large and rather diverse genus, and as with the
division of vetches and peas, several groups of plants within the genus present
themselves as distinct, thus subdividing it into subgenera and sections. It is found that
a useful botanical label in working out where to place a particular species is the
twisting of its style. This may sound rather a synthetic criterion for allocating a
species to a particular group—and indeed this would be the case if it was the only
difference between any two groups—but in the main this is not so: all plants from the
Americas have non-twisted styles, and so do the more primitive Old World species.
No section of the genus contains plants of both sorts. As it happens, both the sweet
pea and everlasting pea have twisted styles. The styles in all Lathyrus species are
oblong in cross section and it is one of the broader flat sides to which the pollen
sticks. In a species in which the style is not twisted, this flat side faces the standard so when the petals of the flower are removed, the style looks broad when viewed from
the front, and narrow when seen from the side. In a species with a twisted style, on
the other hand, the flat side faces the wing of the flower, and the style looks broad
when viewed from the side, and narrow when seen from the front. Figures 2 and 3 are
of a style which is not twisted, Fig. 4 is of a twisted style.
The centre of distribution of the genus is Turkey, with subsidiary centres in North and
South America. There are no representatives in the tropics (except in the mountains
of East Africa), Australia, or New Zealand. It can be assumed that species come from
the Mediterranean region (Europe, Asia Minor, Middle East, North Africa) unless
their origin is specifically mentioned.
In subdividing the genus it is useful to make a chart of important characters contained
in each group :-
It may be a surprise to come across a pea with no leaflets! This refers to two sections:
in one the leaf stalk is broadened so that it looks rather like a blade of grass, and this
acts as the leaf; in the other the bracts at the base of what was the leaf act as the leaf.
The genus can be completely divided into sections, though authorities differ as to the
definition of each section, but in some cases I have subdivided into simpler groups. I
have listed these sections and groups in approximately the order they are thought to
have evolved. I will discuss each in more detail :1 Section Orobus
This is the most primitive section, and it contains a wide variety of characters. The
species range from the short, stocky L vernus which flowers in early spring as the
buds emerge from the ground, the plant eventually attaining a height of about one foot
- to the marsh pea, L. palustris, which looks rather like an everlasting pea (though on
closer inspection, it will be seen that each leaf carried several leaflets and the smaller
mauve flowers were shaped rather differently). L. vernus is quite widely cultivated
and makes a good border or rockery subject. The cultivars vary in colour from blue,
through purple and pink, to white; the leaflets vary from rather like those on a runner
bean (as in cv. Albo-roseus) to long, narrow ones (as in var. flaccidus), but the leaf
never carries tendrils. There are around six, half inch flowers, on a three inch stalk.
Some flower as early as January. One particularly blue form is called L. vernus
“Cyaneus,” and some nurserymen have abbreviated this to L. Cyaneus - unfortunately
a quite different species is known to botanists as L. Cyaneus, - so there is quite a lot of
confusion!
L. aureus, the yellow-flowered counterpart of vernus is also cultivated (sometimes
under the name of luteus aureus, or aurantiacus), but although it produces a profusion
of flowers, it does so later in the season, and is consequently not so popular. L. libani
is not currently cultivated, but has much larger white flowers (more than an inch
across) and is clearly worth consideration. It was once called grandiflorus though the
plant can hardly be confused with the species currently carrying that name (see sect. 7
Eulathyrus). The sea pea, L. japonicus (formerly L. maritimus) also belongs to this
section, but is not so ornamental - likewise L. montanus, bitter pea. The taller species
L. Pisiformis and niger, both with pleasing purple flowers, also belong here and are
seen in cultivation now and again.
2 North American Group (All North American perennials)
The Americans are pleased to call L. ochroleucus, a pale yellow flowered species,
“wild sweet pea”! The real wild sweet pea, if it exists at all, would, of course, be
L. odoratus, so this is rather confusing.
The plant has tendrils, but the stems are not winged and there are numerous leaflets.
The habit is clumpy, rather like the sea pea. Unfortunately, even the Americans find
this a difficult species to cultivate, and although there are several seedlings in this
country, no one has yet managed to get one to flower. Another member of this group
is L. venosus (the “purple wild pea vine”), this has long, strong stalks and purple
flowers which are, unfortunately, all too often lost under large and equally vigorous
leaves. There are up to twenty, half inch flowers on a five to six inch stalk; the plant
grows up to four foot high. From California we have tender species such as
L. splendens (not to be confused with L. latifolius cv. “splendens”) and L. laetiflorus.
These species are said to grow on quite tall woody stems and produce an impressive
display. Until hardy forms are found, though, these remain greenhouse plants. The
marsh pea and sea pea are found in America as well as Europe, and fit equally well
into either of the two groups so far described.
3 Section Platystylis
A more uniform section containing plants generally taller than L. vernus, though still
without tendrils, and it is here that the true L cyaneus belongs. The plants differ from
the North American perennials and most of section orobus in having leaflets with
parallel veins. No species from this section seem to have found their way into
cultivation, but L. variabilis is said to be worthy of consideration, being a strikingly
beautiful plant with pink and lilac bicoloured flowers an inch across and two to seven
in a cluster. A good many species have cream or yellow flowers.
4 Section Pralensis
L. pratensis is a fairly common yellow flowered native. It is called “lady’s slipper,”
“meadow pea”, “meadow vetchling” (not “yellow vetchling” which is L. aphaca) or,
mistakenly, just “vetch.” Its close relatives are perhaps more appropriate for the
garden: L. laxiflorus has downy or glabrous leaves and rich lavender flowers and
forms a fairly neat nine inch clump. L. layardii has larger flowers than laxiflorus
(three quarters of an inch, in clusters of about seven flowers) and might make a still
better garden plant. This is a small section in the genus, the plants surprisingly rarely
cultivated.
5 Section Orobon
One species, placed by itself because it is the only European or North American one
in which the stems are round, rather than angled or winged - otherwise intermediate
between Orobus and Lathyrus and perhaps better considered within the section
Eulathyrus. L. roseus is a small species with large flowers (up to an inch across) of a
carmine pink. The plant stands a foot high, and the flowers are two or three in a
cluster. It is difficult of cultivation - which is a pity, since the flower is very
distinctive. Cultivars of several species have been called “Roseus”, which can lead to
confusion.
6 Section Neurolobus
Another section of a single species, L.neurolobus. A dwarf perennial with one pair of
leaflets (each a half by a sixteenth of an inch), tendrils, winged stem, and blue
flowers. Perhaps of use as a rock garden plant.
7 Eulathyrus Group (perennials of section Eulathyrus)
Here belong all the everlasting Peas - the only everlasting “sweet” pea, L. tuberosus,
or “Fyfield” pea, is the only species, other than the sweet pea to have much scent.
The common everlasting is L. latifolius which can have clear white flowers (cv. White
Pearl, or c.v. albus), pink flowers (e.g. c.v. Pink Beauty), or a rather mundane red.
L.rotundifolius, earlier flowering and with much pleasanter “old rose” flowers is also
cultivated (seven to ten, three quarter of an inch flowers per stalk.) L. undulatus has
been grown in this country for some years under the name L. sibthorpi, and this
flowers still earlier. L. cirrhosus, from the Pyrenees, is another species contributing
its own particular hue of red - it is also the only everlasting to have more than one pair
of leaflets per leaf. L. grandiflorus, the narrow leaved everlasting pea (in reality, the
leaves are sometimes broader than latifolius!) also belongs here - in the wild probably
the largest flowered Lathyrus, L. sylvestris, is the native representative. After the
sweet pea, these are the best known plants in the genus. There is a hybrid in
cultivation: L. rotunadifolius x tuberosus.
8 South American Perennials
Very similar to the European everlasting in several respects, but differing in the
colour of the flowers and their arrangement on the stalk: instead of each flower
coming off singly and alternately one side then the other as at (a), rings of three or
four are attached to the same point on the stalk as at (b).
A plant considered the perennial counterpart of the blue-flowered annual, L. sativus,
was cultivated in the last century as L. magellanicus Lord Anson’s pea. It seems
likely, however, that the plant was one of the other South American species (there are
about fifty) and not the true magellanicus at all. In the 195O’s L. magellanicus, and a
white form (Lord Anson’s White) were in cultivation in the United States. To bring
the hardy blue flowered everlasting, whatever it was, back into cultivation would
seem to be a valuable task. The tender L. pubescens also comes from South America,
and usually has blue flowers. There is a form in cultivation in this country, and it has
flowers large enough to rank with the old sweet peas. A white flowered form has
been recorded from the Andes, and a yellow flowered specimen is said to have been
seen in Chile; the blue form is found in Argentina.
Lathyrus pubescens, note the flowers coming off in rings and the stalks becoming thinner at each junction – a
characteristic of South American species.
Photograph by David Archer
9 Tingitanus Group (L. tingitanus only)
The gypsy sweet pea is an annual. It has quite large pink, red or purple flowers, and is
sometimes cultivated in gardens along with the “old fashioned” sweet peas. It is
curious in being the only species in which the style is twisted the “wrong way.” In all
other species which have a twisted style, looking at the flower from behind, the pollen
sticks to the side facing the right hand wing petal (as in Fig. 4) but in tingitanus, it is
the left hand one. The species comes from Spain and North Africa, notably Tangier.
There are two or three flowers, each up to an inch wide, on a fairly long stalk.
10 Odoratus Group
This is where the sweet pea really belongs. L. odoratus hybridises with L. hirsutus
reciprocally (though with odoratus as the seed parent, the cross is only successful
through stylar amputation). L. hirsutus is another member of this group, which in
common with most species has a “fade character;” as the flowers grow old, the petal
colour changes from a bluish purple to a clear mid blue. This character does not
appear in the sweet pea, but can now be introduced through the odoratus x hirsutus
hybrid - this may be the way to a really clear blue sweet pea flower. L. hirsutus
grows about three feet high, usually with two half inch flowers per stalk. There is a
native form of L. hirsutus, differing from the type in its smaller flowers, and possibly
with a greater concentration of yellow pigment in the petals (making the flowers
appear more of a red colour). Two other members of this group may be of particular
interest to the hybridist: L. chrysanthus and L. chloranthus have golden and sulphur
flowers respectively; since they are closely related to the sweet pea, hybrids might be
possible - further, they are large flowered species (each flower up to an inch across
and two to four in a cluster) and would probably make impressive garden plants in
their own right. As yet, no one has brought these plants into cultivation (visitors to
Turkey and the Middle East, please note!).
11 Cicercula Group (Section Cicercula excluding Odoratus group)
L. sativus, which has astonishingly clear blue flowers, belongs here. Most of the
plants are very leafy and sometimes used for fodder. They tend to form hummocks
like a hemisphere of about two foot radius. Leaflets often of the order of three, by
one inch; the half inch flowers are held singly with stalks about half the length of the
leaves. Species in this section have a wide range of flower colours: brick red in L.
cicera, yellow in L. annuus, orange in L. gorgoni, pink in L. stenophyllus. The pods
tend to be short and stumpy, with large, crinkled seed. L. amphicarpos is worthy of a
mention - a small species, not easily grown, which flowers underground! A few of
the quite large pink flowers will be found above the surface, the rest are, so to say,
self-sowing. L. sativus is quite widely cultivated and its original distribution, if it is a
natural species at all, is not known.
12 Section Orobastrum
This is a section of small annuals with small, inconspicuous flowers (usually red or
yellow) and long, narrow leaflets. The “section is represented in every part of the
world in which Lathyrus appears; in North America by L. pusillus, in South America
by L. crassipes and others, in northern and upland eastern Africa by L. sphaericus,
and in Europe and the Middle East by species such as L. saxatilis and L. inconspicuus,
the latter extending into Asia. Some species show a tendency to become biennials or
even short lived perennials. L. hygrophilus, the species nearest to a tropical plant (its
distribution extends as far as Zaire) is perennial, but until more is known about this
species, it cannot be assigned to this section with certainty. With the possible
exception of hygrophilus, no species is particularly ornamental. The flowers of most
species are a quarter of an inch across, held singly on stalks sometimes as short as a
sixteenth of an inch, and the leaves are generally two by a quarter of an inch.
13 Section Clymenum
A small section of quite ornamental plants. L. articulatus is a tall growing annual
with numerous red and white bicoloured half inch flowers (two or three on a stalk).
L. clymenum is usually a mauve and purple bicolour; sometimes a mauve self. In
some material found in the wild, the distinction between the two species is very
unclear, and they may be better considered as a single species. L. ochrus is much
stouter, with short stalked, solitary, pale yellow flowers and fat pods; perhaps once
used for fodder. The seeds of species in this section are smooth.
14 Section Aphaca
A section of one very variable species. There are no true leaves, the leaf bracts taking
over their function; there are usually a few true leaves on young seedlings. The
vigour of the plant depends on the form grown; cultivars such as “Modestus“ and
“Pseudoaphaca” are dwarf, scrambling a foot or two along the ground, whilst a large,
bright yellow flowered cultivar such as biflorus might grow six foot. The flower
colour varies from golden yellow to cream, and in the vigorous cultivars one or two
half inch flowers are carried on quite a long stalk. It makes a pleasant cut flower to
contrast with violets in a vase and the like. The native form L. aphaca c.v. “Aphaca”)
is rather rare and not particularly appropriate for cultivation.
15 Section Nissolia
Yet another section of just one species. The grass pea looks just like a grass except
when in flower or seed. Again, no true leaves are present, the broadened leaf stalk, or
phyllode taking over the function. The crimson flower is small, (a quarter of an inch)
but on a long stalk and just about noticeable. The species is of little ornamental value.
It is native.
There is no book covering all species in the genus, but there is a good description of
many species in the Flora of Turkey by P. H. Davis. All Lathyrus are hardy, except
the ones I have specifically mentioned as being tender, but some species such as
L. rotundifolius may be damaged by frost unless the top of the root (where the buds
are for next year’s growth) is several inches below soil level.
Lathyrus niger, a primitive species with numerous leaflets, the almost black
flowers approximate to its name.
photograph by David Archer
The seeds of perennials, particularly those in subgenus Orobus are difficult to
germinate compared to most annuals. Particularly amongst the everlasting (section
Eulathyrus) the seeds are very hard and benefit from chipping. Germination can be
slow and uneven, and in primitive species, percentage germination is poor. As a
general rule, the longer the seed takes to germinate, the less vigorous the seedling will
be – seed of L. gmelinii (section Orobus) for instance, may take six months to
germinate - and even in the second year seedlings are only an inch or so high, but this
is an extreme case. Often it is a good policy to sow seeds in two separate pots - one
of chipped seed, one of unchipped with any luck seedlings will soon emerge from the
chipped seed. If the conditions are not quite right, as often happens for one reason or
another, the seedlings may become infected and rot away - the advantage of having
more seeds in a different pot is now evident: the rotting seedlings can be thrown away
leaving the seeds which are probably still dormant in the other pot undisturbed and
uninfected. The unchipped seed will germinate eventually, when conditions are right
- but this may take some time - it is a considerable temptation to discard the pot after
several months and give up hope of anything ever emerging. If there is still no sign of
life by winter, it is often worth putting the pot in a frame and hoping something will
appear in the spring. Once the seedlings are growing well, so long as they do not
become too pot-bound, it is often an advantage to let them die down in their pots and
over-winter in a frame, and plant out the following spring as the first new leaves
emerge.
Annuals are considerably less trouble to grow from seed; they can be treated like
sweet peas - or to save trouble with over-wintering – sown outdoors in the Spring.
For most species little feeding is required.
A note on stylar amputation: a species such as L. hirsutus has a style four millimetres
long, whereas in the sweet pea, it is about ten millimetres. The L. hirsutus pollen
tube, therefore, has to travel only six millimetres to reach the ovary and bring about
fertilisation. When placed on a L. odoratus style it is unable to extend to ten
millimetres or more, so fertilisation fails to take place. By cutting off the sweet pea
style the distance is considerably shortened, and L. hirsutus pollen placed on the cut
end can succeed in bringing about fertilisation. Up to now, the sweet pea has
remained a pure breed being DEVELOPED ONLY FROM THE SPECIES KNOWN
AS L. odoratus; this method gives us the opportunity of changing this and introducing
characteristics new to the sweet pea. From just one species, say L. hirsutus, the
number of such characters must be enormous. In interspecific hybrids it can matter
which way round the cross is made: in breeding including the sweet pea, a more
ornamental plant is likely to be obtained if it is the seed parent. Since the sweet pea
does have a relatively large style, in crossing with other species, stylar amputation is
likely to increase the chances of success. As yet, the best technique to use in the
“amputation” is not apparent; consider two methods :- By cutting the style off at the
base in a transverse or slightly diagonal cut, and applying pollen to the cut surface, the
whole apparatus is left exposed to the air. To avoid possible contamination and
drying out of the surface some sort of protection has to be devised. For this plastic
capsules, or pure, sterile vaseline has been used. The alternative method is to cut the
style longitudinally, down the middle, and to place the pollen at the base of the cleft
so formed; the next stage is to select a hollow straw of the right “bore” and slide a
centimetre section over the style. This holds the pollen in place, prevents drying out,
and reduces the risk of contamination. It probably goes without saying that for this
sort of job a very clean, very sharp blade is required - preferably a scalpel.
Even with stylar amputation, crossing between Lathyrus species is by no means easy,
and where species of similar style length are incompatible, the amputation is of little
help. It is still not clear why some apparently similar species do not cross.
To be continued.