118 STKUCTUEAL BOTANY 



and contains in its interior a number of smaller cells. 

 One of these, situated at the end of the sac towards 

 the micropyle, is the ovum (Fig. 44, 8 and 9, 0), the 

 most important part of the whole structure, because 

 when fertilised it develops into the embryo, or young 

 plant of the next generation. 



The Wallflower has the peculiarity that a number 

 of embryo-sacs, sometimes five or six, are usually 

 formed in the same ovule. This is quite an exceptional 

 character, and it will be more instructive to describe 

 in detail a typical case, in which only one embryo- 

 sac is formed. The development of the ovule will 

 therefore now be traced in the Shepherd's Purse 

 (Capsella Bursa-pastoris), a common weed allied to 

 the Wallflower, which will show us all the essential 

 points. 



Each ovule arises as an outgrowth from the placenta. 

 Several cells belonging both to the epidermis and to 

 the next lower layer, take part in its formation. This 

 outgrowth is the nucellus of the future ovule. When 

 it has attained the form shown in Fig. 44, 1, that cell 

 of the middle row which lies next below the epidermis 

 (e.m in the Fig.) begins to be distinguished from 

 its neighbours by its larger size and more abundant 

 protoplasm. It is termed the archesporium of the ovule. 

 It divides by two transverse walls into a row of three 

 cells (see Fig. 44, 2 and 3). The lowest of the three 

 becomes the embryo-sac (Fig. 44, 3, e). The two sister- 

 cells above it become completely destroyed (see Fig. 

 44, 4), and are, in fact, devoured by the growing 

 embryo-sac. 



