156 ELEMENTS OF STRUCTURAL BOTANY. 



CHAPTER XIX. 



ON THE MINUTE STRUCTURE OP PLANTS THE CELL 



TISSUES TISSU E-S YSTE MS EXOGENOUS AND 



ENDOGENOUS STEMS. 



258. Up to this point we have been engaged in observ- 

 ing such particulars of structure in plants as are manifest 

 to the naked eye. It is now time to enquire a little more 

 closely, and find out what we can about the elementary 

 structure of the different organs. We have all observed 

 how tender and delicate is a little plantlet of any kind 

 just sprouting from the seed ; but as time elapses, and the 

 plant developes itself and acquires strength, its substance 

 will, as we know, assume a texture varying with the 

 nature of the plant, either becoming hard and firm and 

 woody, if it is to be a tree or a shrub, or continuing to be 

 soft and compressible as long as it lives, if it is to be an 

 herb. Then, as a rule, the leaves of plants are of quite a 

 different consistency from the stems, and the ribs and 

 veins and petioles of foliage-leaves are of a firmer texture 

 than the remaining part of them. In all plants, also, the 

 newest portions, both of stem and root, are extremely soft 

 compared with the older parts. It will be our object in 

 this chapter to ascertain, as far as we can, the reason of 

 such differences as these ; and to accomplish this we 

 shall have to call in the aid of a microscope of much 

 higher power than that which has hitherto served our 

 purpose. 



259. First let us examine under our microscope a very 

 thin slice of the pith of the Elder. You see at once that 



