Nichols, A morphological study of Juniperus communis var. depressa. 211 



but disintegrates rapidly after the release of the pollen grains, 

 conveying the impression tliat it is composed of some gelatinous 

 substance. Incidentally it may be noted that similar thickened 

 walls have been observed by the writer in the living pollen niother 

 cells of Picea excelsa, and, as Miss Ferguson intimates, it is 

 probable that these structures are of general occurrence throughout 

 the higher plants. 



During the time which intervenes between the formation of 

 tetrads and pollination the spores round out and increase in size 

 at the expense of protoplasmic material derived mainly from the 

 disorganizing tapetum and inner layer of the sporangium wall. 

 The mature pollen grain (flg. 39) is approximately spherical, about 

 20 ju in diameter, and possesses a thin, transparent intine and 

 a slightly thickened, pigmented exine. The cell contains a Single 

 centrally situated nucleus, and scattered through the cytoplasm, 

 often nearly obscuring the nucleus, are an abundance of large, 

 usually Compound starch grains. 



In regard to the presence of prothallial cells, such as occur 

 in many Gymnosperms, it may be stated with almost positive cer- 

 tainty that these structures are never formed in the Cupresseae. 

 That this is true of Juniperus was first pointed out by Stras- 

 burger (1892). The material studied by the writer shows two 

 peculiar features which at first sight suggest the presence of a 

 prothallial cell. It is observed that shortly after their formation 

 many of the spores contain two nuclei. A thorough search, how- 

 ever, f aus to reveal any indication of mitotic figures, such as would 

 be expected had both nuclei arisen in the pollen grain by the di- 

 vision of the primary nucleus. Moreover the two are apparently 

 alike in every respect, while the spores themselves are invariably 

 somewhat larger than the normal and are frequently constricted 

 at the middle. In view of these facts it seems probable that the 

 binucleate condition is brought about by the failure of a cell wall 

 to develop between two nuclei of a tetrad, and that a consequent 

 twinning of two microspores results. In the same sporangia with 

 these abnormal pollen grains are found others in which there 

 appears what might easily be mistaken for a disorganizing pro- 

 thallial cell, — a dark, lenticular structure, seemingly closely 

 appressed to the wall of the cell. But there is little doubt that 

 this is merely an artificial condition induced by a slight invagination 

 of the spore wall. 



The primary nucleus of Juniperus undergoes no division until 

 after pollination. In this respect Taxus, Cupressus and Juniperus 

 differ from all other Conifers, so far as is known. 



The wall of the mature microsporangium is composed of a 

 Single layer of prismatic cells which are elongated in a direction 

 parallel to the slit by which the anther dehisces. A thin layer 

 of cytoplasm lines the cell walls, and the nucleus in many cases 

 has not yet disorganized. The cell walls are comparatively thin, 

 but on the lateral walls are rib-like bands of thickening (figs. 13, 14) 

 which stain bluish with cyanin, while the wall itself is erythrophilous. 



14* 



