STRUCTURE.] MORPHOLOGY OF TISSUE. 25 



through various stages lying between the two extremes of 

 the small and globular, and the much lengthened, together 

 with an actual perforation of the primary membrane by 

 absorption. To this head belong several formations, first 

 indicated by Moldenhauer, and then correctly and fully 

 described by Mohl, such as the leaf-cells of Sphagnum. But 

 in this more especially consists the difference between cellular 

 tissue and so-called vessels, the latter being nothing more 

 than cylindrical cells, generally situated in the same direc- 

 tion, the terminations lying upon one another, their septa or 

 partitions being perforated in a most varied manner by 

 absorption. 



C. Far more important, however, is the following circum- 

 stance: In the vital process of cell-formation, spiral deposits 

 are by no means at an end with the first layer ; but they are 

 repeated in many cases almost as frequently as the volume 

 of the cell permits. The rule in such cases is, that the suc- 

 cessive strata apply themselves exactly over and upon the 

 first, so that the spaces of the cell-wall, not covered by the 

 first deposit, likewise remain free from all succeeding deposits. 

 Out of this arises the thickening of annular and spiral fibres 

 to such a degree that they resemble plates, placed with their 

 narrow edge on the cell-wall ; as, for instance, in the species of 

 Sphagnum, in the woody cells of Mammillarias, &c. The same 

 class includes all those pitted cells, whose walls are thickened 

 in a stratified manner. But anatomists are acquainted with 

 some interesting exceptions to this rule. It sometimes 

 happens that after the first spiral deposit has been altered 

 by an expansion of the cell, a new layer is thrown down over 

 the whole inner surface, whether fibre or primary cell mem- 

 brane; and since this second layer stands in a different 

 relation to the primary cell membrane from the first, it must 

 also assume a different form, which is the porous. Such 

 formations of distant fibres, between whose convolutions 

 pores are found, are exhibited, in fact, by a number of dico- 

 tyledonous ligneous cells, especially in such plants as are 

 subject to the powerful antagonism of the periods of vegeta- 

 tion and of winter sleep, as, for example, the Yew (Taxus 

 baccata), the Linden (Tilia europaa), the Bird Cherry (Cerasus 



