ANGIOSPERM^E 



The Endosperm 



After fertilization has been completed, the endosperm-nucleus 

 divides. This is sometimes followed immediately by the formation 

 of a division wall (Monotropa, some Araceae), and the embryo-sac 

 is at once filled with a continuous mass of tissue. Much more 

 commonly (Figs. 321, 322) there is a repeated nuclear division re- 

 sulting in many free nuclei lying in the peripheral layer of cyto- 

 plasm, while the centre of the embryo-sac is occupied by a large 

 sap-cavity. Sooner or later, walls are formed between the nuclei, 

 precisely as in the formation of the prothallial tissue in the Gymno- 

 sperms. In case the embryo remains small, this tissue usually com- 

 pletely fills the embryo-sac, and the small embryo is imbedded in a 

 mass of cells, filled with starch or other nutrient matter. If the 

 embryo is large, it often fills the cavity of the embryo-sac at an 

 early period, and the endosperm may remain rudimentary. In such 

 cases, there is a large suspensor developed, and the embryo receives 

 nourishment directly from the outer tissues of the nucellus. Very 

 rarely, as in the Cocoanut, the sap-cavity of the large embryo-sac 

 remains permanently open. 



Sometimes the embryo-sac remains small, and the development 

 of the endosperm is slight. In such cases (Peperomia, Nymphsea), 

 the cells of the nucellus become filled with food materials, and take 

 the place of the endosperm. This tissue is the " Perisperm." 



The Seed 



The integument (testa) of the seed may remain thin, as in the 

 kernel of various stone fruits (Cherry, Peach, etc.), but usually it 

 is hard and the ripe seeds have no further protection. Sometimes 

 there are outgrowths of the integument forming hairs, or wings, as 

 in the seeds of Cotton, Milkweed, Catalpa, etc., and these assist in 

 the distribution of the seeds by the wind. More rarely, as in some 

 Aracese, the oiiter part of the integument is pulpy. 



The Fruit 



In the Angiosperms the stimulus exerted by pollination extends 

 beyond the transformation of the ovule into a seed. Sometimes, at 

 the time of pollination, the ovule is rudimentary (Oak, Orchi- 

 dacese), and it develops during the slow growth of the pollen-tube 

 through the tissues of the pistil. In all cases the carpels are stimu- 

 lated into growth, and keep pace with the development of the enclosed 

 seeds, about which they form a protective envelope. The structure 

 thus formed is the Fruit, using the term in its strict sense. The 



