626 



GERMINATION AND DORMANCY 



hypocofyl- 



cof(jledon 



seed coaf-- 



■plumule 



-hc/pocoi-ij/- 

 p/umu/e 



■coty/edon 

 endosperm 



seecfcoaf 



larged and contain considerable quantities of reserve foods (Fig. 137, J5). 



Some seeds contain a perisperm 

 which represents remnants of the 

 nucellus. The so-called "endo- 

 sperm" in the seeds of gymnosperms 

 is not a true endosperm, but repre- 

 sents the female gametophyte (Fig. 

 137, E). 



Germination of Seeds. — The 

 resumption of active growth on the 

 part of the embr>'0 resulting in the 

 rupture of the seed coats and the 

 emergence of the young plant is 

 known as germination. The seeds 

 of many plants will germinate as 

 soon as ripe if environmental condi- 

 tions are suitable. Pea seeds, 

 for example, sometimes germinate 

 within the pod, corn grains may 

 sprout while still attached to the 

 parent plant and the seeds of some 

 citrus species frequently germinate 

 while still within the fleshy fruits. 

 Seeds of many other species, how- 

 ever, will not sprout until after an 

 interval of weeks, months, or years, 

 even if environmental conditions are 

 favorable for germination. The 

 causes of this condition of dormancy 

 in seeds will be discussed later. 



In nature, germination of seeds 

 usually occurs either at or just be- 

 low the surface of the soil. The 

 latter is more apt to happen in for- 

 ests where seeds, especially smaller 

 varieties, often fall into interstices 

 in the decaying detritus which com- 

 poses the top layer of many forest 

 soils, and are later covered by fall- 

 ing leaves. In the laboratory, non-dormant seeds will usually sprout if 



seecf coat 



"endosperm^ 



embrtjo 



Fig. 137. Structure of seeds. {A) seed 

 of lima bean with the seed coat removed, 

 (6) seed of lima bean with one cotyledon 

 removed, (C) seed of castor bean as seen 

 in longitudinal median section, (D) seed 

 of castor bean with endosperm removed 

 down to the first cotyledon, (£) seed of 

 pinyon (Pinus edulis) as seen in longitudi- 

 nal median section. 



