DICOTYLEDONS. 
^37 
pushing up between them. The cotyledons which thus become exposed to the 
light usually increase rapidly in size, and constitute the first green leaves of the 
plant, which are of simple form {e.g. Cruciferse, Ace?', Cucurbita, Convolvulaceae, 
Euphorbiacese, &c.). If the seed contains an endosperm, the cotyledons do not 
emerge till after it has been absorbed (Fig. 435). 
Many transitional forms occur between the dif- 
ferent modes of germination now described; 
peculiar phenomena sometimes appearing which 
are caused by special vital conditions. In Trapa, 
for example, the primary root is from the first 
rudimentary, and remains altogether undeve- 
loped ; the hypocotyledonary portion of the 
stem lengthens considerably, curves upwards, 
and protrudes a great number of lateral roots 
in rows which fix the plant into the ground \ 
The further developmicnt of the young 
plant may take place by the rapid enlargement 
of the primary axis of the embryo. While the 
axis is growing, generally in an upright direction, 
the shoot which developes from the plumule 
becomes the primary stem of the plant, length- 
ening at the summit, and usually producing 
weaker lateral shoots {e.g. Helianthus, Vicm, 
Fopu/us, Impafi'ens, &c.). When the main stem 
is perennial, it sooner or later ceases to develope 
further at the apex, or the lateral shoots nearest 
to the apex become equally strong. An arbor- 
escent head is thus formed, the main stem or 
trunk becoming denuded by the dying off of 
the lower branches, or the main stem continues 
to grow erect as a sympodium (as in Ricinus, 
the Lime, &c.); or lateral shoots are formed 
at an early period at the base of the primary 
stem which grow as strongly, and thus give rise 
to a shrubby plant. When the axis of the em- 
bryo grows vigorously, the primary root generally 
also grows vigorously in a downward direction^; 
and a Tap-root is thus formed, from which, as long 
as it increases in length, the lateral roots spring 
in great numbers in acropetal succession. When the growth 
Fig. 439. — Almond-seed jj-erniinating, one of the 
cotyledons c' c" being split; tlie letters as in Fig. 438, 
i the first internode strongly developed. 
length of the tap- 
^ [See De Candolle, Organographie Vegetale, PI. 55.] 
2 One of the most remarkable exceptions is afforded by the genus Cuscii*a, which has no 
primary root, the posterior end of the axis penetrating into the ground on germination, but soon 
dying off when the upper filiform portion of the stem has embraced the plant on which it becomes 
parasitic, and has fixed itself on to it by its short suckers ; the plant afterwards grows vigorously 
and branches. 
