36 ROOT. [CH. Ill 



from the rest, which is known as the central cylinder ; the 

 region that surrounds it is known as the cortex, and the 

 layer of cells which limits the cortex, and at the same 

 time limits the outer surface of the root, is the piliferous 

 layer. If the central cylinder is examined a little more 

 closely it will be seen that it presents certain obvious 

 patches imbedded in substance not unlike the cortex in 

 general appearance. These patches are elongated masses 

 or ropes of tissue running longitudinally in the root and 

 known as vascular strands. Both the cortex and the sub- 

 stance in which the bundles are imbedded are made up 

 of tissue which like the parenchyma of the potato tuber 

 is built of cells whose length is not strikingly different 

 from their width. 



In distinguishing the vascular strands from the rest of 

 the root, histologists make use of the word tissue: they 

 speak of vascular tissue and parenchymatous tissue. It is 

 extremely desirable, but by no means easy, to seize and 

 define the meaning of this important term. When a mass 

 of objects is presented to us, our impulse is to classify 

 them ; and the finer elements in vegetable and animal 

 structure are classified into tissues. But not every classi- 

 fication that can be made is a classification into tissues. 

 The conception is to some extent arbitrary, and has to be 

 learned rather than evolved from general principles. It is 

 possible, however, to give certain characteristics common 

 to tissues. 



One such characteristic is that the cells or elements 

 making up a tissue obey a common law of growth. 

 Thus the vascular strands in the root, although made 



