250 



BOTANY OF THE LIVING PLANT 



process is essentially the same. But the pollen-mother-cell, after the first 

 nuclear 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. 197. 



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. 



For the present this brief description must suffice. But later 

 (Chapter XXXI.) the details of behaviour of the nuclei in this important 

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

 and discussed at greater length (p. 467). 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 organisms, even 

 though they differ in many other respects, it may be assumed that it 



