86 



KLEMBNTS OV STRUCTURAL BOTANY. 



embryo consists of a single cotyledon. The radicle is "ieett 

 near the base, and the plumule above. Compa.re an Oat 

 (Fig. 113) with the grain of Corn and make out the corre- 

 sponding parts. In all essential particulars they are alike. 

 118. Comparing the result of our observations with what 

 we have already learned about the Cucumber seed, we find 

 that whilst in the latter there are two cotyledons, in the 

 present case there is but one, and this peculiarity is 

 common to all the plants just examined, and 

 to a vast number of others besides, which are 

 consequently designated MonOCOtyledon- 



ous plants, or shortly Monocotyledons. 



The seeds of this great group may differ as to 

 L the presence or absence of albumen, just as 



the seeds of Dicotyledons do, but in the ntmi- 



ber of their cotyledons they are all alike. 



The Orchids, however, are very peculiar from 



having no cotyledons at all. 

 1 1 9. In addition to the points just mentioned, 



viz : the number of floral leaves, the veining of 

 the foliao'e leaves,the usual absence of distinct petioles, and 

 the sincrle cotyledon, which characterize our second great 

 group, there is still another, as constant as any of these, 

 and that is, the mode of growth of the stem, which is 

 quite at variance with that exhibited in Dicotyledonous 

 plants. In the present group the increase in the thickness 

 of the stem is accomplished not by the deposition of circle 

 after circle of new wood outside the old,_bttt by the pro- 

 duction of new wood-fibres through the interior of the 

 stem generally. These stems are therefore said to' be 



Fig. 113. — Vertical section of Oat gi-ain ; R, radicle ; G, plumule ; C, 

 cotyledon ; A, albumen (or endosperm) ; O, hairs ; T, testa. (Thomi.) 



