276 REPRODUCTION 



tip of the radicle and the root-cap originate by division of the 

 hypophysis, or end cell (h) of the suspensor. 



Ex. 169. — Pull off from an inflorescence of Shepherd's-purse {Capsella) an 

 ovary of a flower from which the petals have just fallen. Open the ovary 

 with needles and remove a few of the ovules : place one or two of the latter 

 in a drop of water on a glass slide and cover with a cover-slip. 



(i) Examine with a low power and sketch the parts of a single ovule and 

 its funicle. 



(2) Press gently on the cover-slip with the end of a lead pencil, so as to 

 burst the ovule : try and find with a low power the embryo and suspensor, as 

 at 3 or 4, Fig. 100, among the contents squeezed out. When found examine 

 and sketch under a high power. 



(3) Repeat the above with ovules obtained from successively older ovaries, 

 and trace the development of the embryo up to the time when well-marked 

 cotyledons and radicle are clearly visible with a low power. 



At the same time as the development of the embryo is going 

 on, changes occur in the embryo-sac and in the nucleus of the 

 ovule. The synergidae and the antipodal cells usually become 

 disorganised and disappear. The secondary nucleus of the 

 embryo-sac, however, fuses with one of the generative cells from 

 the pollen-grain and the compound nucleus arising from such 

 union divides repeatedly until a large number of naked cells 

 are produced, between which cell-walls ultimately arise, the whole 

 then forming a parenchymatous tissue within the embryo-sac ; 

 this tissue is termed the endosperm (e, Fig. loi) and is stored 

 with food on which the embryo lives during its development. 



In wheat, barley, onion and other species of plants the embryo 

 does not disorganise and consume all the endosperm before the 

 seed ripens, so that in these cases a certain amount of endosperm 

 is present in the mature seed (3, Fig. loi). 



In others, however, such as the bean, pea, and turnip, the 

 developing embryo absorbs and utilises practically the whole of 

 the endosperm and the nucellus before the seed ripens, so that 

 mature seeds of these plants contain little or no endosperm-tissue 

 and are described as exendospermous (4, Fig. loi). 



Most commonly the tissue of thp nucellus is disorganised 

 and absorbed during the develppment of the embryo, but in 



