BOTANY OF THE LIVING PLANT 



ssentially the same. But the pollen-mother-cell, after the first 

 division, is itself partitioned by a wall into two cells, in each of which 

 the second nuclear division follows. The final result is thus the same except 

 that the arrangement of the tetrad is not tetrahedral, as it usually is in Dicoty- 

 ledons But the grains lie in pairs, the longer axis of one pair being at right 



Fig. 207. 



A later stage of development of a pollen-sac, showing the young fibrous layer, 

 containing starch. The tapetum (shaded) surrounds the sac in which the tetrads 

 float freely. ( x 100.) F. O. B. 



angles to that of the other. These are minor points ; in all essentials the 

 tetrad-division which produces the pollen is the same throughout Flowering 

 Plants. Figs. 204, 206, 207. 



For the present this brief description must suffice. But later 

 (Chapter XXXV.) the details of behaviour of the nuclei in this impor- 

 tant process of tetrad-division, and chromosome-reduction will be 

 described and discussed at greater length (p. 563). The pollen-grains, 

 as their development shows, are produced from internal tissues of the 

 plant, and are set free by rupture of the superficial tissues. Such 

 bodies are called spores, and the pollen-grains being of relatively small 

 size are called micro-spores. A step in their production is the tetrad- 

 division. The tetrad breaks up later into its four constituent spores. 

 Tetrad-division is a constant feature in the production of spores in all 

 spore-bearing Plants, such as Mosses, Ferns> and Seed-Plants. When a 

 marked feature such as this recurs with constancy in a large group of 



