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A TEXTBOOK OF THEORETICAL BOTANY 



from the fact that if very young embryos are removed from seeds where 

 they normally become curved or otherwise distorted, and are cultivated in 

 freedom, they remain straight. Conditions external to the fruit, for instance 

 gravity, seem to have httle to do with the growth of the embryo, whose 

 symmetry and the relative degree of development of cotyledons and 

 hypocotyl depend on internal and partly on hereditary factors (Fig. 1374). 

 The Onion is a case in point, the embryo being curled in the form of a P, 

 although entirely surrounded by uniform endosperm. Similarly the rolling 



Fig. 1374. — Some unusual forms of embryos. A, Capparis. B, 

 Couroiipita. C, CeratopJiylluni. D, Astelia. E, Tilia. F, 

 Myzodendron. {After Le Maoiit and Decaisne.) 



Up of the cotyledons of Geranium begins when the embryo is still quite small 

 and free from any external pressure. The peculiar form of the embryo in 

 Restio, a biconvex disc on top of the endosperm, is also difficult to attribute 

 to mechanical controls. 



In the Cruciferae the embryos are bent double and their symmetry is 

 an important taxonomic character in the separation of the genera. The 

 chief cases are: i, incumbent , the cotyledons straight and the radicle lying 

 at the back of one of them (Fig. 1375) ; 2, orthoplocous, as above but the coty- 

 ledons conduplicate; 3, spirolobous, as above but the cotyledons folded once; 



4, diplocolobous , as above but the cotyledons folded more than once; 



5, acciimbent, the cotyledons straight and the radicle lying against their edges. 



The degree of development of the cotyledons in the seed is correlated 

 with the extent of the reserve tissues. The smaller the amount of the latter, 

 the larger, in general, are the cotyledons, which are themselves utilized as 

 reserve stores. The limiting case is that of the absence of any endosperm 



