ANATOMY 25 



ground tissue parenchyma may be modified into several 

 kinds of cells fcr different purposes, and in young stems, 

 which are green, the outer layers of the parenchyma 

 usually contain minute green grains, the chlorophyll 

 granules which play such an important part in the manu- 

 facturing of food. Often mixed with the parenchyma, 

 in regular strands 'or groups, are thick-walled scleren- 

 chyma cells, and their position in the stem is almost 

 always that which is mechanically most advantageous. 

 In stems there is generally a pith of soft parenchyma 

 cells, and round that the vascular tissues are arranged 

 in groups, each group composed of a strand of wood 

 and a strand of bast. As the stem grows these separate 

 strands of vascular tissue are joined to form a ring by 

 secondary formations of wood and bast. Instead, 

 therefore, of the central, solid strand of Vascular tissue, 

 as in the root, the stem is characterised by a hollow 

 cylinder which is formed round a central pith. In some 

 few stems of the higher plants, outside this cylinder an 

 endodermis sheath like that in the root can be seen, and 

 this is a fact which is of much theoretical importance. 



There are many views as to the real meaning and 

 origin of the woody cylinder, and the one which seems 

 to be best supported by facts considers the hollow 

 vascular cylinder to be the descendant of a solid strand 

 not unlike that in the root, the central cells of which 

 lost their character as wood cells and became simple 

 parenchyma. The stems, which are preserved for us 

 as fossils, seem to support this view, though at first 

 sight it may sound rather far-fetched to say that the 

 cells of the parenchyma on one side of the vascular 

 strands have a different value from those on the other 

 side of the same strands. 



Probably one of the most powerful influences in the 



