THE WALLFLOWER 61 



cambium better after we have considered the develop- 

 ment of tissues in the growing points. 



ft. Other Tissues of the Central Cylinder 



The central cylinder consists of the vascular 

 bundles and of parenchyma, which is called conjunctive 

 tissue because it unites the bundles together. That 

 part of the conjunctive tissue which lies inside the 

 ring of bundles, and is called the pith, is of very 

 simple structure. It is made up of large, rather 

 elongated, square-ended cells, which retain their living 

 contents, and usually form starch granules. The 

 interfascicular tissue, constituting the medullary rays, 

 is similar to the pith, but its cells are smaller. It 

 is important, because its cells divide up to form the 

 interfascicular cambium, which unites the cambial 

 layers of the separate bundles into a continuous ring 

 of actively multiplying cells (see Figs. 20 and 21). 

 Owing to the presence of this layer of cambium, the 

 stem is able constantly to form new tissues, and thus 

 to grow indefinitely in thickness (see below, p. 90). 



Surrounding the whole ring of vascular bundles 

 on the outside is a layer of thin-walled cells, the 

 pericycle (pc in Figs. 20, 21, and 22). This layer 

 forms the outer limit of the central cylinder. Beyond 

 this we come to the cortex. 



Those cells of the conjunctive tissue which border 

 on the vascular bundles are generally rich in grape- 

 sugar, dissolved in their cell-sap. It is through these 

 cells that the carbohydrates pass on their way from the 

 leaves to other parts. 



