DICOTYLEDONS. 



637 



\ 



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. Cruciferae, Acer, Cucurbila, Convolvulaceae, 

 Euphorbiaceae, &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 development 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. Helianihus, Vici'a, 

 Fopu/us, Impah'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 in length of the tap- 



FlG. 439.— Almond-seed germinating, one of the 

 cotyledons c' c" being split ; the letters as in Fig. 438, 

 i the first inteniode strongly developed. 



* [See De Candolle, Organ ographie Vegetale, PL 55,] 



^ One of the most remarkable exceptions is afforded by the genus Cuscuta, 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. 



