24 MORPHOLOGY OF TISSUE. [BOOK i. 



b. Simple or compound fibre ; still a rather considerable 

 expansion of the cell; slight, or no cohesion with the 

 cell membrane : hence spiral vessels, with broad convo- 

 lutions, capable of unrolling. 



c. Simple or compound fibre ; extremely slight expansion 

 of the cell membrane, generally intimate cohesion with 

 it; hence narrowly wound spiral vessels capable of 

 unrolling, false tracheae, and in part the ringed and 

 scalariform vessels of older writers. 



d. Compound fibre ; moderate expansion of the cell, cohe- 

 sion of the convolutions inter se in some places, generally 

 also with the cell membrane. This produces the whole 

 series of so-called branched and ramified reticulate spiral 

 vessels. Hereto likewise belongs a portion of the striped 

 and scalariform vessels of the older writers. 



In these last, as well as in all the preceding, the law, that 

 the more intimately the fibre coheres with the cell membrane, 

 the less this can expand, appears to obtain. 



2. But if the cell has, at the time when the spiral deposi- 

 tions have begun to form, already attained its complete 

 expansion, a new and highly remarkable circumstance comes 

 into action, namely, that the formation of air- vesicles on the 

 outer wall of the cell, between it and the adjacent ones, 

 precedes the origin of the deposits; and the convolutions 

 forming, closely lying one upon another, and in most cases 

 rapidly cohering inter se, separate from one another cleft-wise 

 at the place which internally corresponds to those air^ vesicles. 

 Since this process can be followed very far, and yet cannot, 

 on account of the minuteness of the parts, be observed in 

 some formations otherwise exactly similar, sound analogy 

 advises us to extend it to all porous textures. This slit, 

 which is in general narrow, is often rounded by deposited 

 matter which causes the pore to appear the rounder the more 

 the cell is developed; the longer and more slit-like, the 

 younger it is. To this division belong all porous cells and 

 vessels, and likewise a portion of the young striped and 

 scalariform vessels, which at that time only differ from those 

 called pitted or porous by the length of the fissure. 



B. A further advance consists in the progress of the cell 



