554 



PHANEROGAMS. 



the young pollen-cells become free, separate, and float in the granular fluid which 

 fills up the cavity of the anther; and within this they now attain their definite 

 development and size. The fluid being thus used up, the mature pollen-grains finally 

 fill up the cavity of the anther in the form of a powdery mass. 



[The ripe pollen-grain of Angiosperms has been found in many cases to contain 

 two nuclei ^ It appears that when the pollen-grains have become isolated from each 

 other, the nucleus of the grain undergoes division into two, one larger, the other 

 smaller. The smaller nucleus travels to the wall of the grain and becomes invested 

 by protoplasm, thus constituting a primordial cell, which, in some cases, is cut off 

 from the rest of the grain by a wall of cellulose : the larger nucleus remains as the 

 nucleus of the larger cell of the pollen-grain. The smaller nucleus may divide once 

 or twice, thus giving rise to a group of cells; the large nucleus does not divide: the 

 form of the nuclei varies very much. These processes resemble those which have 



Fig. 379.— Mother-cell of the pollen oiCucurbita Pepo; sg the outer common layers of the mother-cell itl the act 

 of being absorbed ; sp the so-called ' special mother-cells,' consisting of masses of layers of the mother-cell which 

 surround the young pollen-cells ; they also are afterwards absorbed ; ph the wall of the pollen-cell ; its spines grow 

 outwards and penetrate the special mother-cell ; v hemispherical mass of cellulose on the inside of the pollen cell- 

 wall, from which the pollen-tube is afterwards formed ; / the protoplasm contracted ( < 550). (The preparation was 

 obtained by making a section of an anther which had lain for some months in absolute alcohol.) 



been described as taking place in the pollen-grains of the Gymnosperms : the small 

 cell (or the cells derived from it) evidently corresponds to the * vegetative ' cells in the 

 grains of Gymnosperms and in the microspores of the heterosporous Vascular 

 Cryptogams.] The pollen-tube is formed from the large cell : it is developed as a 

 protuberance of the intine, which perforates the extine at certain definite spots that 

 have usually been prepared beforehand. The spots where this perforation takes place 

 are often more than one, or even very numerous (Fig. 380 a, 381 o)\ yet, notwith- 

 standing the possibility of the formation of this number of pollen-tubes from one grain, 

 only one usually grows to an extent sufficient to effect impregnation. Independently 

 of the structure of the extine itself which has already been mentioned, the external 



^ [Hartig (in Karsten's Botan. Untersuch. III. 1866) was the first to observe two nuclei in a 

 pollen-grain : he found them in the grains of Tradescantia, Campa7mla, CEnothera, Lilium, Clematis, 

 Allium, etc. His observations have been extended by Strasburger (Ueber Befruchtung und Zelltheilung, 

 1878) and by Elfving (Jenaische Zeitschrift, 1877, and Quart. Journ. Micr. Sci., XX. 1880.)] 



