THE STAMEN 145 



as poUinia, the abutting walls are said to remain uncutinized. The ex- 

 tent to which the mother-cell wall takes part in the formation of the 

 pollen-grain wall by secretion or by modification and adnation to the 

 spore wall probably varies greatly. The wedge-shaped projections that 

 furrow the cytoplasm have been claimed to be formed within a 

 gelatinous inner layer of the mother-cell wall. 



The spores are variously placed in the tetrad (Fig. 64); those formed 

 by the simultaneous method are usually in tetrahedral positions; those 

 formed by the successive method, in isobilateral positions. The decus- 

 sate arrangement is occasional; the linear, rare. The T position is also 

 rare and is probably abnormal. More than one type of arrangement 

 may occur in a species. 



The spore mother cell usually forms four spores but may form more 

 or less than four — two, when the second division fails to take place; 

 three, when one of the first pair of cells fails to divide; more than four 

 (pohjspory), occurring chiefly in hybrids, where divisions are irregular 

 or incomplete. Of four formed, one only may mature; the other three 

 degenerate, often remaining as three vestigial nuclei beside the surviv- 

 ing spore inside the mother-cell wall — Cyperaceae (Fig. 60). Polyploid 

 nuclei and other complex nuclear conditions may result from failure of 

 wall formation in meiosis and from fusion of spore mother cells. 



The Microspore Wall. The microspores, at first enclosed by a delicate 

 wall, which may be deposited upon the inside of the mother-cell wall 

 wherever the spore abuts upon it, lie within the mother-cell wall, usu- 

 ally completely filling it. The mother-cell wall may hold the tetrad to- 

 gether for a time, even permanently when the pollen grains remain in 

 tetrads; or it may gelatinize and merge with the spore wall to form a 

 part of the pollen-grain wall. Sometimes, it is apparently absorbed. 

 The walls of the microspores enlarge, expanding and thickening by 

 intussusception. The exine — outer layer — is thick and its function is 

 protective, both mechanically and physiologically. It is adapted to 

 changes in volume of the grain — expansion and contraction with changes 

 in humidity. The intine — inner layer — is thin and delicate and also 

 readily adapts itself to changes in size of the pollen grain. The wall 

 may become very thick and complex in structure, and the outer layer 

 become sculptured with projecting ridges, spines, and granules ("orna- 

 mentations"). Inner and outer layers, usually increased in wall thick- 

 ness as the spore enlarges, are distinct; the inner is cellulosic, the outer 

 cutinized. In the Epacridaceae, however, the wall of the young spore 

 is thick and becomes thinner as it enlarges. The tapetal plasmodium 

 aids, in large measure, in the increase of size of the spores and, 

 especially, in the thickening of the wall and the building of the projec- 

 tions. When this thickening and ornamentation is completed, the spore 



