FOVILLA.—FORMS AND SIZES OF POLLEN. 263 
the large cell produces the pollen-tube. (See ‘ Reproduction of 
Gymnospermia,’ in Physiological Botany.) 
2. Contents of the Pollen-cells.—The matter contained within 
the coat or coats of the pollen-cell is called the fovilla. This 
consists of a dense, coarsely-granular protoplasm, in which are 
suspended very small starch granules, and what appear to be 
oil globules. As the pollen-cell approaches to maturity, the fovilla 
becomes more concentrated, and contains less fluid matter, and 
more granules. Some of these granules are not more than 
about 35455 of an inch in diameter, while the largest are about 
zong OF sooo Of an inch. They vary also in form, some being 
spherical, others oblong, and others more or less cylindrical 
with somewhat tapering extremities. When water is applied to 
the granular contents they become opaque. When viewed under 
a high magnifying power, the starch granules at certain periods 
(especially at the period of the dehiscence of the pollen) exhibit 
a very active tremulous motion, moving to and fro in various 
directions and appearing as if repelled by each other. This is 
simply molecular motion, analogous to that of all other very 
minute particles when suspended in a liquid. The fovilla is 
without doubt the essential part of the pollen-cell, but the office 
it performs will be explained hereafter. 
3. Forms and Sizes of Pollen-cells.—Pollen-cells are found of 
various forms. The most common forms appear to be the 
spherical (jigs. 73 and 567), and oval (fig. 568) ; in other cases 
they are polyhedral, as in Chicory (Cichoriwm Intybus) and 
Sonchus palustris, or triangular with the angles rounded and 
enlarged (trigonal), as in the Evening Primrose (nothera 
biennis) and plants commonly of the order Onagracee (fig. 573), 
or cubical as in Basella alba, or cylindrical as im Tradescantia 
virginica, while in Zustera they are threadlike or of the form 
of a lengthened tube or cylinder, and other forms also occur. 
It should also be noticed that the form of the pollen is materi- 
ally influenced according as itis dry or moist. Thus the pollen- 
cells of the Purple Loosestrife (Lythrwm Salicaria) and some 
species of Passion-flower are oval when dry, but when placed in 
water they swell and become nearly globular: this arises from 
osmotic action taking place between the thickened fovilla and the 
water, by which some of the latter is absorbed, and the pollen- 
cells consequently distended. Again, when spherical pollen-cells 
are exposed to the air for some time they frequently assume a 
more or less oval form. In size, pollen-cells vary from about 
545 to zs, Of an inch in diameter ; their size, however, like their 
form, is liable to vary according as they are examined in a dry 
state or in water. 
We have already stated that when the pollen-cells are placed 
in water they become distended in consequence of osmotic action 
taking place between their thickened contents and the sur- 
rounding fluid. If this action be continued by allowing the 
