i2 4 A TEXT-BOOK OF BOTANY. 



nucleus with the surrounding cell-contents, constitutes the tube- or 

 wall-cell of the antheridium. 



Development of Ovule and Megasporangium (nucellus). 

 The ovule at first develops as a small protuberance on the inner 

 surface of the ovary, after which it differentiates into (a) a stalk 

 or funiculus by which it is attached to the ovary, the tissue to 

 which it is attached being called the placenta; and (b) an upper 

 portion which becomes the ovule proper. The differentiation of 

 the tissues is in a general way as follows : ( i ) The cells beneath 

 the epidermis in the apical portion of the ovule go to make up the 

 megasporangium (nucellus) ; (2) the peripheral cells from below 

 the nucellus give rise to the integuments ; and (3) while the integu- 

 ments are developing the archesporium or mother cell of the 



x. 



FIG. 81. Development of male gametophyte in an Angiosperm. I, pollen grain 

 (microspore) which has divided into the mother or generative cell (v) and a larger tube-cell 

 with nucleus (sk) ; II, appearance of pollen on treatment with osmic acid showing the 

 separation of the generative cell (v) from the wall of the pollen grain; o.at the right giving 

 a view of the generative cell with the nucleus embedded in the hyaline protoplasm; III, 

 showing the development of the tube-cell into the pollen tube which contains the two male 

 cells (nuclei) or gametes formed by the generative cell. After Elfving. 



embryo-sac (megaspore) is being formed within the nucellus near 

 the apex. 



Female Gametophyte.- -The archesporium divides into two 

 cells, the lower one of which repeatedly divides, finally giving 

 rise to the embryo-sac which is sunk in the tissues of the nucellus. 

 The nucleus of the embryo-sac divides and redivides until 8 cells 

 are produced (Figs. 82 and 83), which are separated into the fol- 

 lowing groups: (i) Three of the cells form a group lying at the 

 apex, the lower cell of the group being the egg or egg-cell, the other 

 two cells being known as synergids or helping cells. (2) At the 

 opposite end of the sac are three cells, known as antipodal cells, 

 which usually develop a wall of cellulose and do not seem to have 

 any special function. (3) Near the centre of the sac are the two 

 remaining nuclei, which unite to form a single nucleus, from 



