GERMINATION 



935 



the plumule to light, most of the food is unnecessary and 

 may gradually decompose in the ground ; but if condi- 

 tions for autophytic nutrition are less favorable, much or 

 even all of the food may be used by the seedling. 



Aspects of germination external to the seed. The 

 earliest conspicuous external index of germination 

 is the rupture of the testa and the protrusion of 

 the embryo. The time necessary for such pro- 

 trusion, after the seeds have been exposed to 

 proper germinative conditions, varies from one or 

 two days (as in lettuce or mustard) to some weeks 

 or months (as in the date). Small seeds germinate 

 more quickly, as a rule, than do large seeds, prob- 

 ably because the foods are digested more quickly 

 through easy access to water and oxygen. Starchy 

 seeds commonly germinate more quickly than do 

 fatty seeds, and much more quickly than do 

 seeds with " reserve cellulose." The rupture of 

 the testa, which usually becomes much softened 

 and weakened by the absorbed water, may be 

 effected by the growing radicle or by the coty- 

 ledon, as in many monocotyls. Sometimes the 

 embryo emerges through thin spots, as in the 

 coconut, or pushes out a loosely fastened plug 

 of tissue. Usually the radicle is the first part of 

 the embryo to protrude, and this is doubtless ad- 

 vantageous, since most seeds contain enough food 

 for considerable growth, while all of the water 

 must come from without. Often (as in the 

 cocklebur) the radicle is so situated that it is the 

 first part of the embryo with which the entering 

 water comes in contact, 1 and the absorption of 

 water from the soil by the young root system 

 usually is well initiated before the external de- 

 velopment of the plumule becomes prominent. 



In some cases the cotyledons remain in the soil, especially where 

 these organs are the chief seat of accumulated food, as in oaks and 



1 The significance of the position of the radicle in the seed of the cocklebur is seen from 

 the fact that if a bit of the testa is removed near the tip of the cotyledons, growth begins 

 at that point, the radicle then being the last part to react. 



Monocotyl seedlings : 

 1229, an onion seed- 

 ling (A Ilium Cepa), il- 

 lustrating epigaean 

 germination ; note the 

 curvature of the coty- 

 ledon (c) whose tip re- 

 mains within the seed 

 (s), acting as an ab- 

 sorptive organ; 1230, 

 the seedling of a date 

 palm (Phoenix dactyl- 

 ifera) in longitudinal 

 section ; note the re- 

 markable cotyledon (c) 

 whose axis elongates 

 upon germination ; one 

 end of the cotyledon 

 (5') remains within the 

 seed, acting as an ab- 

 sorptive organ ; the 

 other end continues 

 for a time to enclose 

 the plumule (/) and 

 the radicle (r). From 



