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 

 Coniferse, 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 hoyau 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 diiference 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 Asclepiadese 

 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 



