SPERMATOPHYTA 



393 



fusion of four or more nuclei of the embryo sac, with the addition of the 

 male nucleus. 



Following fertilization, the petals and stamens wither and drop off, and 

 often the sepals do likewise. As the ovules are transformed into seeds, 

 the ovary enlarges to form a fruit. Thus normally fertilization provides 

 a stimulus that has far-reaching 

 effects. 



Endosperm. Typically endo- 

 sperm arises from a triple-fusion 

 nucleus, division of which usually 

 precedes that of the fertilized egg. 

 Sometimes each of the nuclear di- 

 visions is accompanied by the for- 

 mation of a wall, so that a tissue is 

 formed at once (Fig. 33(3A). More 

 commonl}^, however, the formation 

 of endosperm is initiated by free- 

 nuclear division (Fig. 336fi) . These 

 free nuclei are usually parietally 

 placed but sometimes fill the em- 

 bryo sac. Unless the endosperm 

 is absorbed by the embryo while 

 still in the free-nuclear stage, wall 

 formation then takes place, often 

 simultaneously throughout the en- 

 dosperm, resulting in a compact 

 tissue without intercellular spaces. 

 Reserve food becomes stored in its 

 cells, generally in large quantities. 

 This may be deposited as hemicel- 

 lulose on the cell walls, which often become very thick, as in the date 

 and many other palms. In some angiosperms, as in many of the Podo- 

 stemaceae and Orchidaceae, the endosperm is absent or greatly reduced, 

 often being represented by only a few free nuclei. The endosperm may 

 persist in the seed as a food-storage tissue or may be entirely absorbed by 

 the developing embryo. 



The integument or integuments of the ovule are transformed into the 

 testa of the seed. The nucellus is almost or entirely destroyed during the 

 development of the seed, but in such forms as the Centrospermales it 

 persists and gives rise to a tissue, called perisperm, which becomes the 

 chief food-storage region. In the Nymphaeaceae the seed contains both 

 endosperm and perisperm. 



A B 



Fig. 336. Two methods of endosperm 

 formation. A, Silphium laciniatum, nu- 

 clear division followed immediately by wall 

 formation, X300; B, FritiUaria biflora, 

 endosperm arising by free-nuclear division, 

 X 150. In A, the fertilized egg (above) has 

 not yet divided; in B, it has divided once. 



