THE FLOWER 119 



Spillman (1909) states that wheat has 40 or more chromosomes. 



Nakao (1911) gives 8 as the haploid number, and Bally (1912) found 

 the same in the wild T. dicoccoides and T. vulgare. 



Sax (1918) found approximately 28 in the first division of the fertilised 

 egg of T. durum (Kubanka) and approximately 40 in the fusion-nucleus 

 of the embryo-sac. 



Sakamura (1918), working chiefly with root-tips, obtained the follow- 

 ing results : 



Diploid 



No. 



T. dicoccum . 28 

 T. monococcum . 14 



This series with 7 instead of 8 as the basic haploid number is remark- 

 able. These numbers require further investigation. 



At the first division the pollen mother-cell is bisected into two equal 

 hemispherical halves with a cell wall between each ; the next division 

 occurs in a plane at right angles to the first, and is also followed by the 

 formation of a partition wall. The four pollen-grains thus produced lie 

 in the same plane and remain for a short time within the delicate wall of 

 the mother-cell ; later, each develops its own wall, which is differentiated 

 when mature into an outer thick cutinised coat, the exine, and a thin 

 cellulose lining, the intine of the spore ; in the exine is the pollen-tube 

 pore previously described. 



The pollen mother-cells when rounded off and the tetrads produced 

 from them are arranged in regular order as a layer round the inner wall 

 of the loculus ; the free pollen-grains become scattered irregularly through- 

 out the cavity. 



The pollen-grains germinate while still in the unopened anther, and 

 before the protrusion of the pollen-tube producing the vegetative or tube- 

 cell possessing an oval nucleus, measuring 7 ^ x 8 /z, and two slender male 

 gametes, each of which is curved and pointed at one end and measures 

 10-12 /A x 2 [t, (Fig. 103). 



During the development of the pollen-grains changes occur in the cell 

 layers constituting the walls of the anther loculi. The middle layer, 

 which is the thinnest, collapses, its walls and scanty contents appearing 

 as a narrow line round the outer boundary of the tapetum. 



The tapetum consists of oblong parenchymatous cells square in trans- 

 verse section with abundant cytoplasm and containing at first a single 

 large nucleus. About the period when the sporogenous cells undergo 

 their first division the nuclei of the tapetum divide, the two daughter 

 nuclei in each cell persisting with no wall between them until the layer 

 disappears, when the pollen-grains are perfectly developed (Fig. 98). 



