430 STRUCTURE AND FUNCTIONS OF THE POLLEN. 
circumstances under which dehiscence may take place :—in 
water no gut being protruded, but the fovilla scattering itself 
irregularly through the liquid; while on the stigma, the tube is 
formed in consequence of what he conceives to be a vital action. 
I find that under the influence of acids, the contents of the 
pollen are protruded in many instances in a form closely re- 
sembling a gut or tube, but that in pure water, when rupture 
takes place, the fovilla diffuses itself through the fluid without 
any order. « 
— My next observation explains how this happens; for I 
find in the transparent pollen of Monocotyledones, as well as 
of the Rosacea, Leguminose, &c., the addition of acid renders 
them instantly opaque; and I am thus led to conclude that 
the fluid in which the fovilla floats, is coagulated by these re- 
agents. Now, when the external membrane, of the pollen 
dehisces by pores, it is easy to- understand why the coagu- 
lated contents, forced through these small round apertures, 
should assume a tubular or gut-like appearance. In the 
Liliacee, Smilacee, Butomee, and several allied families, the 
debiscence is by a suture; and in those cases, upon the addi- 
tion of acid, the external membrane peels off, leaving the 
contents of the original form: while I have sometimes found 
species, such as Butomus umbellatus, and Iris fetidissima, which 
have naturally opaque pollen, burst in water in a manner 
similar to their allies under the influence of acid, and I have i. 
always found these varieties capable of reddening litmus. © > 
An appearance which might seem to support ient - 
niart’s hypothesis, is frequently seen in Gentianee, Tropeolee, 
Linee, Plumbaginee, Polemoniacee, Labiate, and some other — 
. families. The pollen of these plants when placed in water 
. or acid, may be observed to form on their surfaces little vesi- — 
cles, which are at first transparent, but become in acid soon 
opaque. These vesicles are evidently formed by the protru- 
m of an inner membrane of the pollen, through pores in 
r one; but when they burst, the usual results take 
zater, the fovilla becomes scattered, if in acid, it 
