314 THE STRUCTURE OF ECONOMIC PLANTS 



semi-desert conditions, where warm or hot days, cool nights, and 

 a relatively dry air both night and day generally are encountered." 



Although it is believed by many investigators that tripping is 

 essential to effective pollination. Brink and Cooper (4) point out 

 that "Under greenhouse conditions in the winter, tripping is prac- 

 tically indispensable to seed setting, but in the field many pods may 

 arise from untripped flowers." Carlson (5) noted in Utah that 

 only about 8 per cent of the flowers were tripped, while 14 per cent 

 set pods; and the semi-arid climate is undoubtedly an important 

 factor in the production of seed in that region. Factors which 

 may tend to decrease the amount of seed produced are variations in 

 the behavior of the stigmatic membrane as a block to effective 

 pollination; abortion of a relatively high percentage of the pollen; 

 and, less frequently, abnormal positional relationships between the 

 stigma and the anthers. An additional factor has been suggested 

 by Martin (xo), who attributes blasting of the seed, in some cases, 

 to an arrested development of the embryo, due to the inability of 

 the plant to furnish proper water and food supply; as well as to 

 pathological conditions to which the seed is more susceptible under 

 drought conditions. 



MicROSPOROGENEsis. — In the development of the anther, there 

 is an early differentiation of a hypodermal column of archesporial 

 cells one or two layers in thickness. These in turn produce the 

 primary parietal cells and the primary sporogenous tissue by the 

 formation of periclinal walls. The parietal cells again divide 

 periclinally and the outermost layer redivides, so that the anther 

 wall consists of an epidermal layer, two middle layers, and the 

 tapetum. The primary sporogenous cells divide several times to 

 form a column of microspore mother cells. According to Reeves 

 (i6), the tapetum differentiates rather slowly and its cells remain 

 uninucleate, while the outer parietal cells become elongated 

 radially and form an endothecium. Later the tapetum and inner 

 layer of parietal tissue disintegrate and their contents are absorbed 

 by the developing microspores; so that, at anthesis, the walls of 

 the locules consist of only the epidermis and endothecium. Fol- 

 lowing reduction division and wall formation, which occurs by 

 furrowing rather than by the formation of cell plates, the micro- 

 spores increase in size until they are several times their original 

 diameter. The wall of the slightly three-lobed mature pollen 

 grain consists of an intine and exine layer with three germ pores, 



