294 BOTANY 



ground, serve to absorb water by holding it in their substance 

 or drawing it in hygroscopically (cf. Mistletoe berries). Fruit-walls, 

 by their spongy nature, may also serve as water-carriers (ripe fruits of 

 Tropaeolum, Potcrium spinosum, Medicago terebellwm). 



The germination of seeds, once securely lodged in the soil, may 

 begin immediately or after a longer or shorter period OF rest. 



The seeds of many Conifers do not germinate for several years. Some plants 

 again, in addition to seeds which germinate in the first year, produce others which 

 require a longer rest ( Trifolium pratense, Robinia Pseudaeacia, Cytisus Laburnum, 

 Reseda lutea, etc.). Even under favourable circumstances such seeds do not 

 germinate until a definite length of time has elapsed. Germination may be de- 

 layed also by external conditions, and the vitality of the seed may still be retained 

 for years. Thus, for example, on the removal of a forest from land that had been 

 under cultivation for forty-six years, Petek found that a great variety of field- 

 plants at once sprang up as soon as the requirements for their germination were 

 restored. 



Germination, according to the observations of Klebs, is introduced 



by true processes of growth, which result in THE rupture OF the 



seed-coverings. This is effected either by the growing radicle, or, in 



many Monocotyledons, by the cotyledon. In other seeds enclosed within 



a shell, the bursting of the shell through the growth of the endosperm 



or cotyledons precedes germination. In cases where the shell is very 



hard and does not consist of two halves easily separable by internal 



pressure (as in Cherry-stones), special places are often provided for 



the egress of the young seedling. At the end 



of a cocoa-nut, for example, such points of egress, 



behind the thinnest of which the embryo will be 



found emerging from the endosperm, are very 



easily seen. Through the extremely hard, thick 



shell of another Cocoa-palm, Corns, lapidea, there 



are three long germinal pores, while the seedling 



of Acrocomia sclerocarpa has only to push a loosely 



fastened plug out of the thick shell of the seed 



(Fig. 221). Similar contrivances are found in 



the upper part of the the case of Pandanus, Canna, Typha, Potamo- 



imitofAwocomiasdero- geton, and many Dicotyledons (Tetragonia expansa, 



sheii'l p, the plug which Medica 9o, and some species of Onobrychis and 



is pushed out of the PortuldCa). SEEDLINGS PENETRATE THE SOIL by 



shell by the germinat- me ans of the elongation of the primary root, or of 



ing embryo, K ;E, endo- ,1-1 , i 1 - ,, .., 



sperm. (After Pfitzer.) the hypocotyl, or also, as is the case with many 

 Monocotyledons, through the movements of the 

 geotropic cotyledons. After the descending part is firmly attached to 

 the soil, by either root-hairs or lateral roots, the upward growth com- 

 mences. In this process the cotyledons may either remain within 

 the seed or unfold above ground. The first is often the case where 

 the cotyledons are full of reserve material (Phaseolus multiflorus, Aes- 



