322 BOTANY PART i 



once or after a resting period (dependent in part on delay in swelling 

 due to hard seed coats, partly on the time needed for the growth of 

 the embryo) commences to germinate. In some cases the germina- 

 tion (both of seeds and of fungal spores or pollen grains) is dependent 

 on the presence of special substances or of the host plants (Orobanche, 

 Tozzia, spores of Bulgaria) in other cases of light (Viscum, Drosera, 

 Pitcairnia mdidifolia, Veronica peregrina, Nicotiana). In the case of 

 fern-spores the action of light can be replaced by warmth, and in 

 moss spores by the presence of sugar in the solution. The action 

 of light is on the other hand inhibitory in the germination of 

 Acanthostachys strobilacea and Phacelia tanacetifolia ( 123 ). 



The seeds of many Conifers do not germinate for several years ; those of the 

 Ash and Hornbeam in 2 and those of Euphorbia cyparissias in 4-7 years. Some 

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

 which require a longer rest (Trifolium pratense, Robinia pseudacacia, Cytisus 

 Laburnum, Reseda lutea, Euphorbia exiyua, species of Dianthus, etc.). Even 

 under favourable circumstances such seeds do not germinate until a definite 

 length of time has elapsed. Treatment with strong sulphuric acid may in such 

 cases increase the permeability of the seed-coat and hasten germination. Germina- 

 tion may be delayed 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, PETER found that a great 

 variety of field-plants at once sprang up as soon as the requirements for their 

 germination were restored. The plants developed under such circumstances are, 

 however, often weakly ( 124 ). 



Germination, according to the observations of KLEBS, is intro- 

 duced by true processes of growth, which result in THE RUPTURE OF 

 THE SEED COVERINGS ( 125 ). This is effected either 

 by the growing radicle, or, in many Monocoty- 

 ledons, by the cotyledon. In other seeds enclosed 

 within a shell, the bursting of the latter by 

 the growth of the endosperm or cotyledons pre- 

 cedes germination. In cases where the shell is 

 very hard and does not consist of two halves 

 easily separable by internal pressure (as in 

 Walnuts and Cherry-stones), special places are 

 . tl . often provided for the egress of the voung 



FIG. 251. Section through x 



the upper part of the seedling, 

 fruit of A crocomia sdero- 



carpa. S, The hard g ucn pl aces are often present in the thick and hard 

 S hell;P, the plug which seed . coats of m lms At the end of a Coco . nut f or 



is pushed out of the * . r 



shell by the genninat- example, three points of egress, behind the thinnest of 

 ing embryo, K;E, endo- which the embryo will be found embedded in the endo- 

 sperm. (After PFITZER.) sperm, are very easily seen. Through the extremely hard, 

 thick shell of another Coco-palm, Cocos lapidea, there 



are three long germinal pores, while the seedling of Acrocomia sclerocarpa has 

 only to push ti loosely fastened plug out of the thick shell of the seed (Fig. 



