190 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



the internal framework of the stem, much in the 

 same way as in the netted cylinder of ferns. The 

 deposition of silica in the cells of Equisetum no 

 doubt greatly adds to the rigidity of the stems and 

 branches. Judging by their shapes, we should infer 

 that the ancient Calamites could bear more exposure 

 than the Lepidodendra. The latter, we are inclined 

 to think, were not adapted for growing in exposed 

 situations ; the larger species were probably confined 

 to low elevations. 



III. Phanerogams. The Coniferce have been re- 

 ferred to in an earlier part of this paper, but before 

 passing from the Gymnosperms w^e may notice the 

 remarkable WeJwitschia. In this odd-looking plant 

 the leaves may be said to be absent, their function 

 being performed by the large persistent leathery 

 cotyledons. There is properly no stem, but only the 

 greatly thickened hypocotyledonary axis. The plant 

 in fact retains in its adult state the form commonly 

 seen in the embryonic stages of flowering-plants. A 

 plant of this pattern could never attain great size, 

 and yet the close resemblance of the embryos and 

 seedlings of the higher plants to Welicitschia w^ould 

 seem to point to some such type as the progenitor 

 of existing phanerogams. The leaves of Monocoty- 

 ledons call for notice, because, with the exception of 

 the palm-leaf, they do not generally exhibit that 

 subdivision of the leaf-blade so common among 

 Dicotjdedons. The iDarallel veins, which constitute 

 the vascidar system of the leaf, are closely related 

 to the vascular system of the stem; indeed the leaf 

 is only a stem flattened out to adai3t it for its 

 peculiar function. In most cases the flattening 

 occurs in a horizontal plane, but in Iris Ave have it 

 taking place A^ertically. The i:)arallel veins appear 

 to have prevented the division of the leaf from tak- 

 ing place in a pinnate manner; and, where the 

 necessity for diminishing resistance has arisen, the 

 end has been gained either by palmate division, as 

 in palm leaves, or by elongation into the linear or 



