GEBMINATIOy OF THE ONION 325 



And where is the slit from whicli the plumulu emerges ? Onion seeds 

 germinate slowly, but the process may be hastened by soaking them 

 in warm water for a few hours. Be sure that the seeds are fresh. 

 Perhaps the pupil can have access to an onion patch in very early 

 spring. In a warm room and with fresh seeds, the entire process 

 of germination may be completed in two weeks. 



396. In the squash we found two cotyledons, 

 but in the onion there is only one. This fact is 

 of great significance, because it affords a natural 

 character for the classification of. plants. 

 We have already (376) divided all 

 flower- bearing plants into two cate- 

 gories, the angiospermsB, with distinct 

 ovaiy, and the gymnospermge, without 

 distinct ovary. We may again divide 

 the angiosperms into two classes or 

 groups, — the dicotyledons (two seed- _r;; 

 leaves) and the monocotyledons (one ^^"^ ^^' 



The whip-lash. 



seed-leaf) , 



397. These characters derived from the number 

 of cotyledons, are not mere arbitrary or conven- 

 tional standards for the classification of plants, but 

 they are correlated with other features. For ex- 

 ample, the dicotyledonous plants are more or less 

 clearly characterized by making their increase in 

 diameter of stem by means of rings or layers of 

 growth upon the outside of the woody cylinder and 

 underneath the bark ; they are, therefore, some- 

 times called exogens ("outside growers"). They 



