CHAP. II. 
POLLEN. 
157 
branous and extensible: the last of these opinions is enter- 
tained by Adolphe Brongniart and Amici. Brown says that 
the existence of an inner membrane is manifest in several 
Coniferae, in which the outer coat regularly bursts and is 
deciduous; and further, he considers that the structure in 
Asclepiadese, as discovered by Mr. Francis Bauer, furnishes 
the strongest argument in support of the opinion of the exist- 
ence of two membranes. In parts of such extreme minute- 
ness and delicacy of structure, a point of this kind cannot be 
determined by sections ; for the sharpest knives in the most 
skilful hands will only crush the grain of pollen into a shape- 
less mass. The evidence of the existence of an internal 
membrane is derived from the appearance of a thin trans- 
parent coating round the fovilla when it is emitted upon the 
stigma, and which is sometimes extended to a considerable 
length. Its existence having been called in question, 
Adolphe Brongniart was induced, in his examination of the 
anther, to pay particular attention to the circumstance ; and 
he declares that, " in all the pollen that he has examined with 
care, after it had been a greater or less space of time upon 
the stigma, he has found a tubular appendage, of variable 
length, formed of an extremely thin and transparent mem- 
brane, which evidently proceeded from the interior of the 
grain of pollen, either through an accidental opening, or 
through a special passage formed in the external membrane." 
(See Plate IV. figs. 34. to 38.) He calls this pollen tube 
the boyau or intestine. Notwithstanding the precise manner 
in which this is stated, it has nevertheless been doubted by 
some whether the boyau or pollen-tube is any thing more than 
mucus surrounding the fovilla when emitted. Brown, in 
1828, declared his difference in opinion from Brongniart as 
to the existence of a membrane forming the coat of the pollen- 
tube; but, in 1831, he states, in another place, that several 
arguments may be adduced in favour of Brongniart's opinion, 
that the pollen-tubes belong to the inner membrane of the 
grain ; and he particularly cites the structure in Asclepiadeae 
as favourable to the opinion. Fritzche confirms the state- 
ment that two coats are actually present, and asserts that the 
only exceptions he has discerned are in plants that flower 
