THE PROBLEM 49 



Beans, Walnuts, Orchids and Water Plantains. 

 The former are called albuminous seeds, the lat- 

 ter exalbuminous old-fashioned terms based 

 on a comparison between the endosperm of a 

 seed and the white of an egg. Now, albumi- 

 nous seeds are common to Gymnosperms and 

 Angiosperms, though in Gymnosperms the de- 

 velopment of the endosperm is absolutely dif- 

 ferent, for it is formed before fertilisation; 

 exalbuminous seeds, however, are peculiar to 

 Angiosperms and limited to certain families 

 among them. The significance of this point will 

 appear when we come to the fossils. 



The reproductive characters of the Angio- 

 sperms are the most important. In the vege- 

 tative organs stem, leaf and root there is 

 such a vast range of structure that we cannot 

 profitably frame any short description common 

 to the whole. One characteristic point is that 

 the wood is, as a rule, more complicated in 

 structure in Angiosperms than in Gymnosperms 

 or any other plants. In particular, large con- 

 tinuous vessels, formed of rows of cells fused 

 together end to end, are characteristic of the 

 wood of Angiosperms, though vessels of a differ- 

 ent kind occur in other groups of plants. The 

 comparatively simple Gymnospermous wood, 

 consisting essentially of tracheides (long, pitted, 

 water-conducting cells with pointed ends) and 



