122 THE WHEAT PLANT 



Trio-three antipodals divide and increase in size at the expense of the 

 nucellus, producing a convex mass in contact with the wall of the embryo- 

 sac on the placental side a short distance from the chalazal end (Figs. 101, 

 105). The antipodal complex in the mature sac usually consists of 6 

 to 10 laige cells, although Kornicke mentions 36 or more. Each measures 

 from 3aTto 60 ^ in diameter and contains much vacuolated cytoplasm and 

 a large nucleus ; after fertilisation is accomplished the cells are gradually 

 disorganised, their contents being apparently utilised in the formation 

 of the endosperm ; their large nuclei, which may reach a diameter of 

 40 fj, with nucleoli 16 p or more across, are the last to disappear. 



During the development of the flower, the stamens and their pollen- 

 sacs are for a time considerably in advance of the carpel and ovule. 



At the period when the anther possesses a well-defined single longi- 

 tudinal row of sporogenous cells surrounded by two parietal and one 

 epidermal zone, the carpellary cavity is open above, and the mass of 

 meristem, from which the ovule is formed, shows neither integument nor 

 archesporium. 



When the first or inner integument is beginning to develop and the 

 archesporium is clearly defined, the sporogenous cells of the anthers have 

 divided and a double longitudinal row of closely fitting cells is seen in the 

 pollen-sacs ; the tapetum has also been formed, so that at this stage there 

 are four layers round the sporogenous cells. 



The midiment of the second or outer integument of the ovule appears 

 before the archesporium divides. The nucellus becomes more and more 

 enclosed by the integuments, and two successive mitoses of the arche- 

 sporium occur, resulting in the production of a tetrad row of megaspore 

 mother-cells. Pollen-grain tetrads, or rounded separate pollen-grains 

 with single nuclei and more or less cutinised walls, are now found in the 

 anthers ; the cells of the tapetum are still somewhat square in transverse 

 section, but the cytoplasm is reduced and much vacuolated. 



When the embryo-sac is ready for fertilisation, each pollen-grain con- 

 tains the tube-nucleus and two gametes, the tapetum has become com- 

 pletely disorganised, and the endothecial layer shows its characteristically 

 thickened walls. 



V 



ANTHESIS AND POLLINATION 



Anthesis, or opening of the flower, follows the escape of the ear from 

 the upper leaf-sheath in five or six days, although in some instances it 

 occurs on the first day, or may be delayed till the ninth after the appear- 

 ance of the ear. The development of the stamens and gynaecium in the 

 same flower proceeds at the same rate, and the pollen is ready to be shed 

 from the anthers when the stigmas become receptive. 



