24 



MORPHOLOGY 



of the plumule ; that is, the leaves of the embryo are alternate. 

 But the caulicle is usually very short, and there is no external 

 mark by which its limits may be distin- 

 guished from the cotyledon, until germi- 

 nation has begun. For a type of it, the 

 embryo of some aquatic or marsh plants 

 \i \ // ffSjffl ma y be taken, where it forms the whole 

 11 kernel of the seed (Fig. 50-53), and 

 80 n. 52 us the structure can be made out antecedent 

 to germination. It is understood by supposing that the cotyle- 

 don, which forms its principal bulk (the caulicle being only the 

 very short thickish base) , is convolute around a short 

 plumule, and the margins concreted, except a minute 

 longitudinal chink at base, out of which the growing 

 plumule protrudes in germination. The embryo of 

 Iris may be similar in structure, but no distinction 

 of parts is visible. It is very small in proportion to 

 the size of the seed, the kernel being mostly albu- 

 men, a supply of food, from which the germinating 

 embryo draws the materials of its growth. When 

 this takes place, either the cotyledon or the whole 

 embryo lengthens, its lower part is pushed out of the 

 seed, a root forms at the free end of the excessively 

 short caulicle, and the plumule develops from the 

 other in a series of one-leaved nodes, the internodes 

 of which remain so short that the leaves continue 

 in close contact, the bases of the older successively 

 enclosing the inner and younger. (Fig. 55.) Here, 

 therefore, the cotyledon mainly remains in the seed, 

 and the seed remains underground (hypogaeous) . 



40. It is somewhat different in the Onion, which 

 has a similar embryo, except that it is longer, and 

 the cotyledon is curved in the albumen of the seed. 

 The first steps are the same as in Iris ; but as soon 

 as a root is formed and embedded in the soil, the 

 cotyledon lengthens vastly more, into a long and 

 filiform green leaf, which, taking an erect position, 



FIG. 50. Seed of Triglochin palustre ; the rhaphe, leading to the strong chalaza at the 

 summit, turned towards the eye. 51. The embryo detached from the seed-coats, showing 

 the longitudinal chink at the base of the cotyledon ; the short part below is the radicle. 

 62. Same, with the chink turned laterally, and half the cotyledon cut away, bringing to 

 Tiew the plumule concealed within. 53. A cross-section through the plumule, more 

 magnified. 



FIG. 54. Section of seed of Iris, enlarged, showing the small and apparently simple 

 embryo at the base of the albumen. 55. Germinating seed and seedling of the same, of 

 natural size. 



