FORM OF THE PLANT-CELL. 49 



(here belong the so-called porous vessels of the wood) ; or in one 

 part of the cell it becomes changed to rings, whilst in another part 

 it becomes spiral, netted, or, what more frequently happens, it re- 

 mains entirely porous. 



To this point too little attention has been given. We know in this 

 relation only the last modification, the mixed tubes (tubes mixtes) of 

 Mirbel. Here, however, belong also the so-called porous vessels of our 

 dicotyledonous woods, which, in the manner in which they are spoken of 

 in books as tubes formed of an entirely porous membrane, certainly do 

 not exist. All these so-called vessels are only so far porous as they 

 touch one another ; as, where they project on the wood-cells, these walls 

 are often almost entirely homogeneous, and exhibit scarcely a trace of 

 pores. This may be easily seen if the individual cells are isolated by 

 means of nitric acid. When these vessels are arranged in radial rows, 

 but never or very seldom when they lie laterally on one another,, if we 

 cut them directly across we shall find very evident porous walls, but 

 never or extremely seldom are they to be seen by a longitudinal section. 

 This is the case in the Coniferce, where the pores predominate (but not 

 exclusively) on the side next the medullary rays ; or in the Hibbertia 

 volubilis, where they appear, on the contrary, towards the pith and the 

 bark, seldom towards the side of the medullary rays ; so that the two 

 other sides in these cases have a homogeneous development. 



20. The process of depositing layers is often repeated during 

 the life of a cell. a. Each successive layer is generally deposited 

 accurately upon the preceding, ring upon ring, spiral upon spiral, 

 porous layer upon porous layer, b. But in some less frequent 

 cases the deposit takes place according to the circumstance of the 

 cell ; so that when, through extension, the cell-fibres become sepa- 

 rated, the extension causes the deposit of a porous layer. Ordinarily, 

 also, the direction of the spiral in the following layer is the same 

 as in the foregoing, but in some cases the direction of the next 

 spiral is directly opposite that of the first. 



The first condition mentioned above is very common, and rings are 

 often found so very much thickened that they have only a little hole in 

 the centre ; and as they do not increase so much in breadth, they appear, 

 when perfectly formed, like thin discs with a hole bored 

 through them. They are seen in the Cactece, as Opun- 

 tia cylindricay Melocactus, Mammillaria, &c. This 

 occurrence is also very frequent in porous cells, so that 

 the cavity of the cell is reduced to a scarcely visible 

 point. Such cells are very common in plants, and a 

 single layer may be easily seen upon a transverse section. 

 The pores of the deposit layers become gradually con- 

 verted into canals (fig. 32.). Such canals frequently 

 approach each other, and at last unite, so that the inner 

 layer is much less porous than the outer (fig. 33.). 

 With these may be compared the elegant formations in the so-called 



32 Transverse section of three liber-cells and some parenchyma-cells in the China 

 reffia (Cinchona scrobiculata Humb.). The liber-cells show very clearly the deposit 

 layers and the porous canals. 



