38 MARGARET C. FERGUSON 



accounts for its haviug been overlooked by previous writers. It 

 is not shown at all in the series of figures, recently published by 

 Coulter and Chamberlain ('01), illustrating the development of 

 the pollen-grain in Pinus Laricio. 



The various tests commonly used in determining the nature 

 of the cell-wall have been applied to the young pollen-grains 

 as well as to the special spore-mother-walls. These tests show 

 that the outer wall of the pollen-grain is clearly of the nature 

 of cutin, as has been demonstrated by Strasburger. Both the 

 innermost wall of the microspore, and of the pollen-grain, as 

 also the wall of the special spore-mother-cells, respond to the 

 reaction for cellulose, but not in a very marked manner. If 

 they are of the nature of cellulose there would seem to be an 

 admixture of some other substance, but I have not succeeded 

 in obtaining entirely satisfactory results regarding the nature of 

 these inner, prominent walls. Tests thus far have been made 

 with "fixed" material only; further experimentation along 

 this line will be made when fresh material is at hand. 



During the season of growth, the nucleus of the microspore 

 always remains close against the convex or dorsal side of the 

 spore, occupying a central position along this wall. As is 

 usual in cell-development, the microspore-cell attains full size 

 before any mitoses occur within it, and there is never any fur- 

 ther increase in the size of this cell after the inception of the 

 first division. The fully developed microspore is, therefore, 

 the exact counterpart, so far as size is concerned, of the mature 

 pollen-grain. Compare fig. 54, plate V, with fig. 65, plate 

 VI. During the development of the microspore, the cytoplasm 

 which at first was uniformly distributed in a rather loose net 

 work, becomes more closely reticulated and at the same time 

 less abundant in proportion to the size of the cell. At the 

 maturity of the spore the cytoplasm is largely distributed about 

 the nucleus from which strands extend outward in a radial man- 

 ner and end in the ectoplasm. In 1898 the microspores of 

 Pinus Strobns were ready to leave the mother-cells on May 30, 

 they had attained full size on June 7, and on June 10 the pollen- 

 grains were fully mature. 



