134 STRUCTURAL BOTANY 



g. The Endosperm 



Kecent discoveries have shown that a doiible act of 

 fertilisation takes place within the embryo-sac ; in 

 addition to the fertilisation of the ovum, the second 

 generative nucleus from the pollen-tube unites with 

 the secondary nucleus of the embryo- sac, itself, as we 

 have seen, the product of a nuclear fusion. To this 

 subject we shall return when we come to our next 

 type, the Lily, p. 186, Fig. 86). The fertilised nucleus 

 of the embryo-sac undergoes repeated divisions, and 



around the daughter- 

 nuclei cells are form ed. 

 In this way a little 

 tissue is produced, 

 called the endosperm, 

 a name given to any 



FIG. 48. Embryo-sac of Elder (Sam- tissue thus formed in 



bucus), which has just become filled ,1 , f ,-, 



up with endospeJ by cell-division. the interior of the 



en, endosperm ; em, embryo. (After embryo-sac, and Ollt- 

 Hegelmaier.) Magnified about 250. - , ,, f , 



side the embryo, after 



fertilisation. But in the case of the Wallflower and 

 all its immediate allies, such as the Cabbage, Turnip, 

 Shepherd's Purse, Watercress, and plenty of other 

 well-known plants, the greater part of the endosperm 

 is only a temporary structure. It never attains any 

 considerable size, and is to a great extent used up as 

 food by the growing embryo before the seed is ripe, 

 only the outermost layer being persistent. Seeds 

 which when ripe contain nothing besides the embryo 

 are called exalbuminous, because they contain no albu- 

 men, an old-fashioned name applied, to the endosperm 



