Spiral Formations in the Cells of Plants. 37 



least two such bands present*, whose extremities at the end 

 of the cells pass into one another, and in most cases, even 

 very early, cohere inter se to a single one. 



Hence, then, proceed all the varied formations of the cells 

 and vascular walls, according to the different influence of the 

 following momenta. 



A. The most essential circumstance, in my opinion, upon 

 which is also founded the division of all these textures into 

 two large principal groups, that of the Spiroidea (I borrow 

 this expression, which is very useful, from Link), and that of 

 the porous formations, is the following : 



Either the cell has, at the time when the thickening of its 

 wall by spiral deposition commences, already attained its 

 complete expansion, or not. 



I. Let us, in the first place, consider the latter case. Here, 

 then, a second momentum becomes of importance ; it is the 

 cohesion both of the fibre and the cellular Avail, and of the coils 

 of the fibre inter se ; at the same time, therefore, the number 

 of fibres is likewise of value. 



a. Simple fibre (double in the sense above stated). The 

 cell still expands considerably from the instant of its origin ; 

 some convolutions cohere early, others tear asunder : annular 

 vessels (of which a more detailed description bklow) In this 

 case the fibre is generally not at all, or but loosely united with 

 the cellular membrane. 



b. Simple or compound fibre, a still rather considerable 

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

 membrane : spiral vessels with broad convolutions, capable of 

 unrolling. 



c. Simple or compound fibre, extremely slight expansion 

 of the cellular membrane, generally intimate cohesion with it : 

 narrowly wound spiral vessels capable of unrolling, false tra- 

 cheae, and in part the striped and scalariform vessels of older 

 writers. 



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

 sion in some places of the convolutions inter se, generally 

 also with the cellular membrane : the whole series of the 

 forms of the so-called ramified spiral vessels to the reticulate. 

 Hereto likewise belong a portion of the striped and scalari- 

 form vessels of the older w r riters. 



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

 the more intimately the fibre coheres with the cellular mem- 

 brane, the less this can expand, appears to obtain. 



* Corresponding to an ascending and descending current of the mucous 

 formative substance. 



