LY THRUM 
467 
6-merous. The axis (‘calyx-tube’) is hollow, generally tubular. The 
sepals are valvate, and frequently possess an epicalyx, formed, as in 
Potentilla, of combined stipules. Petals crumpled in bud, sometimes 
absent. Sta. inserted (often very low down) on calyx-tube, typically 
twice as many as sepals, but sometimes fewer or 00 . Ovary superior, 
with simple style and usually capitate stigma ; it is 2 — 6-loc., at the 
base at least, rarely i-loc. with parietal placenta. Ovules usually 00 , 
anatropous, ascending. The firs, of Lythrum (q.v.) and others are 
heterostyled. Dry fruit, usually capsular. No endosperm. Chief 
genera: Peplis, Lythrum, Cuphea, Lagerstroemia. Benth. -Hooker 
include Oliniaceae, and place it in Myrtales ; by Warming it is 
placed in Myrtiflorae. 
Lythrum Linn. Lythraceae. 23 sp. temp., in damp places; 2 in Brit., 
Z. Salicaria L. (purple loosestrife) and another. The 6-merous firs, 
are solitary or in small axillary dichasia like those of Labiatae. Each 
has 12 sta. in two whorls of different length, and the style again is of 
different length to any of the sta. Three forms of fir. occur, each on 
a separate plant; they are distinguished as long- mid- and short- 
styled firs., according to the length of the styles. The diagram illus- 
trates the arrangement of parts (S = stigma, 
A = anthers, B = base of fir.), as seen in 
S s 
A3 
A3 
side view of the fir. Darwin (Forms of 
A 2 
S 2 
A, 
Firs.) was the first to show the meaning 
A, 
Aj 
Si 
of this trimorphism. It is evident that an 
B 
B 
B 
insect visiting the firs, will tend on the 
long- 
mid- 
short- 
whole to transfer pollen from A 3 to S 3 , 
styled 
styled 
styled 
A 2 to S 2 , A x to S 1? rather than from sta. 
of one length to style of another, for the insect will always enter 
these zygomorphic firs, in the same way and to the same depth. The 
sta. and style project so far that an insect can alight directly upon 
them. Darwin showed by a long series of experiments that the best 
results are obtained by pollinating S 3 from A 3 , or S x from A x , &c., 
i.e. by crossing two plants. The number of seeds thus obtained 
is much greater and their fertility higher than if S 2 or S x be fertilised 
from A 3 , or any other such union be made. Fertilisation of a stigma 
by sta. of corresponding length Darwin terms legitimate , by sta. of a 
different length illegitimate fertilisation. The offspring of illegiti- 
mate fertilisation are few in number and have the sterility and other 
sexual characters of hybrids. 
A curious point, as yet entirely unexplained, is that here, as in 
nearly all other heterostyled plants, the longer the sta. the larger the 
pollen grains, and the longer the style the larger are the papillae of 
the stigma. 
The whole subject of heterostylism is still much in the dark. We 
know nothing beyond the facts, and the cause, even the advantage, of 
the phenomenon remains obscure. We can only class it as one of 
3° — 2 
