33^ CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



than the intervening periblem cells and also than those of 

 the plerome. They become separated from the epidermis 

 by periclinal walls of more or less regular formation (fig. 

 10) ; but the hypodermal cells between the large periblem 

 cells described divide by anticlinal walls only. This cell- 

 formation results in the lobed condition characteristic of 

 anthers. 



A cross-section of an anther in which the loculi are visible 

 shows one conspicuously large cell surrounded by two 

 series of periblem cells and the epidermis. Of these three 

 concentric cell series the middle one forms the endothecium, 

 and with the epidermis persists until the anther is ripe. The 

 inner series, according to Koernicke, divides by periclinal 

 walls to form the tapetum and a middle layer, both of which 

 are resorbed in the later stages of the development of the 

 anther. This point, however, I did not see in the oat. The 

 large cells which occupy the middle of each loculus and 

 form, as longitudinal sections show, a single row, constitute 

 the archesporium. Later these divide by radial walls and 

 thus give rise to the pollen mother-cells. A median longi- 

 tudinal section through a loculus in which the pollen mother- 

 cells are formed (fig. 11) shows two rows of spore 

 mother-cells, each of which reaches to the walls of the 

 loculus. 



The history of the division of the mother-cells into tetrads 

 I did not follow. Koernicke gives a detailed account of 

 this for the wheat, and we may suppose that the history of 

 spore formation in the two nearly related genera is very 

 similar. 



The mature pollen spore is enclosed by two integuments, 

 the inner of which is very delicate. The spore is densely 

 filled with coarse granules of starch. Three bodies that 

 stain strongly with the reagent used (Haidenhain's iron- 

 alum-haematoxylin) are present in the spore. 



The largest is oval and stains less strongly than the two 

 spindle-shaped ones. These three are respectively the 

 vegetative and the two generative nuclei. Some difficulty 

 was encountered in distinguishing the nuclei from the cyto- 

 plasm in the generative cells, and it is probable that the 



