280 



BOTANY OF THE LIVING PLANT 



not strictly maintained. Meanwhile the embryo-sac grows rapidly, 

 and the large central vacuole is surrounded by a thin peripheral film of 

 cytoplasm, in which the free nuclei are embedded (Fig. 221, iii.-vi.). 

 Partition-walls are formed later, isolating each nucleus in its own cyto- 

 plasmic area. Sometimes several may be present in a single cell, but 

 when this is so they commonly fuse together. The embryo-sac is thus 

 lined internally by a flattened layer of uni-nucleate cells, surrounding a 

 large central vacuole (vii.). These cells then grow inwards, and divide, 

 encroaching on the central cavity : this they ultimately fill with the 



compact tissue of the endo- 

 i sperm, which embeds the 



embryo. Sometimes, how- 

 ever, a central cavity may 

 still remain. This is the case 

 in the Coco-Nut, where the 

 cavity is filled . with the 

 " milk," which is actually 

 vacuole-fluid, while the white 

 flesh is the tissue of the endo- 

 sperm, which has not filled 

 the very large embryo-sac. 



The above description applies 

 to ordinary types of ovule. But 

 there is a considerable variety 

 of detail in the behaviour of the 

 embryo-sac and its contents after 

 fertilisation. Frequently the first 

 division of the fusion-nucleus is 

 followed by a cell-wall dividing 

 the sac into two chambers ; this 

 is seen in some Monocotyledons, and in many Dicotyledons, especially among 

 the Gamopetals. Sometimes the development proceeds no further ; but usually 

 divisions may be continued in one or the other, or in both of the chambers. 



More marked modifications are connected with the nourishment of the 

 embryo-sac. In relatively primitive types like Myosurus the sac merely 

 increases greatly in size, encroaching upon the surrounding tissues, which 

 make way for it. Their cells collapse and their substances are absorbed into 

 the growing sac, which acts thus parasitically upon them (Fig. 221, ii.). 

 But in more specialised types, such as the Gamopetals, the nursing of the 

 embryo-sac is more exact. They have only a single integument, while the 

 sac so n crushes the small nucellus out of existence. The innermost layer 

 of the integument then abuts on the growing sac, and forms an epithelial 

 jacket of prismatic cells (Fig. 222, ./.}. This serves as a permanent 

 nourishing tissue, which acts till the embryo-sac is well advanced. 



FIG. 222. 



Median section of an ovule of Rhinanthus minor, 

 showing haustoria. (After Balicka-Iwanowska.) s=sus- 

 pensor. em -embryo, e.s. = embryo-sac, fun. =funiculus. 

 n.j. -nutritive jacket, nl. = nutritive tissue, c.h. =cha- 

 lazalhaustorium. m.k. -micropylarhaustorium. 



