122 INTKODUCTION TO CRYPTOGAAriC BOTANY. 



more to say under another groujD, which consists in the union of 

 the endochromes of two contiguous vesicles, in consequence of 

 which, a single large spore is formed in the connecting tube. 

 These spores are mostly globular, but like the fronds themselves, 

 exhibit great differences of surface, so that, apart from the 

 matrix, they may be taken for the sporidia of truffles, or other 

 heterogenous bodies. In many cases, they have no resemblance 

 whatever to the parent frond, and though mixed with them, 

 would never be suspected to have a common origin, until the 

 union of two vesicles, and the consequent spore, should be ob- 

 served. These, however, when once formed, are propagated 

 by division, exactly after the fashion of the ordinary cells, and 

 in the third generation acquire their normal form, which they 

 may contLaue to propagate for years, without ever forming a 

 true spore. They differ most materially from the following 

 tribe, in the constant absence of a siliceous coat,* and though 

 it has been attempted to shew that it does exist in the fossil 

 species, more accurate observations have detected deflection of 

 the spines, without fracture, which is inconsistent with such a 

 notion. In one particular group (Closterium), distinguished for 

 the more or less entire outline, which is either straight or arcu- 

 ate in most of the species, there is a peculiar organ at either ex- 

 tremity, consisting of a hyaline or straw-coloured cell, containing 

 a multitude of active molecules, possibly of sexual import. Many 

 of the species of Closteriuin are remarkable for the close-set 

 longitudinal striae with which the cells are marked. Traces of 

 striee have, however, been found in a Tyndctrldea, by Mr. Jenner, 

 and in Tiresias, by Mr. Bowerbank. Desmidiacece are confined 

 almost exclusively to fresh water, though Mr. Thwaites has 

 found one or two species in brackish water, but in every case 

 such species only as also occur in fresh water. Little can be said 

 about then- distribution, for few extra-European species are 

 known; and as we travel to the South of Europe, the species 

 appear to diminish in number, their central point, perhaps, being 

 the South of England. There was a trace merely of some Clos- 

 terium in the Himalayan collections of Dr. Hooker and Dr. 



* It is conjectured, however, that a small portion of silicate of iron 

 does exist in some species of Closteria, oommimicating a ferruginous tinge. 



