TISSUES. 169 



changes set in, as we have seen, at a very early stage, and 

 eventually all the cells assume permanent forms, some 

 developing in one way, others in quite a different way, 

 according to the function of each particular part. So that 

 in any given plant we find tissues, or groups of cells, of 

 very various kinds, and very different arrangements of 

 these tissues in different cases. By examining sections 

 taken in succession from the growing point backwards, 

 every degree of change from meristem to permanent tissue 

 may be made out. 



286. In the growing parts of all plants, in the pulp of 

 fruits, in the pith, in the green parts of leaves, and in 

 the entire substance of many plants of low organization, 

 we find tissue composed of short and comparatively thin- 

 walled cells, to which the name parenchyma has been 

 given. On the other hand, in the substance of wood, in 

 the inner bark, in the petioles and veins of leaves, &c., 

 we meet with tissue consisting of long, pointed, and 

 overlapping cells, and known as prosenchyma. That of 

 the wood is fibrous tissue, and that of the inner bark is 

 the bast, specially characterized by the extraordinary 

 length and flexibility of the cells. Sclerenchyma and 

 collenchyma have already been referred to. In the- 

 former the cells are commonly, though not always, short; 

 while in the latter they are usually long, but the ends 

 are not pointed. 



287. Cells have been described which are characterized 

 by peculiar markings on their walls. When such cells 

 stand end to end, the cross-partitions commonly dis- 

 appear, with the effect of forming long tubes, generally 

 of larger diameter than the other cells with which they 



