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 m.uch. These processes resemble those which have 
Fig. 379. — Mother-cell of the pollen of Cucurbita Pepo; sg the outer common layers of the mother-cell ifi 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 ( x 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 0)) 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 Bo tan. Untersuch. III. 1866) was the first to observe two nuclei in a 
pollen-grain : he found them in the grains of Tradescantia, Campanula, CEtiothera, Lilium, Clematis, 
Allium, etc. His observations have been extended by Strasburger (lieber Befruchtung und Zelltheilung, 
1878) and by Elfving (Jenaische Zeitschrift, 1877, and Quart. Journ. Micr. Sei., XX. 1880.)] 
