98 ELEMENTS OF STRUCTURAL BOTANY. 



CHAPTER XV. 



ox THE MINUTE STRUCTURE OF PLANTS EXOGENOUS AND 



ENDOGENOUS STEMS FOOD OF PLANTS. 



160. Up to this point we have been engaged in 

 observing 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 

 eleineutdry structure of the different organs. We have 

 all observed how tender and delicate is a little plantlet 

 of any kind just sj)routing 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 consis- 

 tency 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 inirpose. 



161. If a small bit, taken from a soft stem, be boiled 

 for a while so as to reduce it to a pulp, and a little of 

 this pulp be examined under the microscope, it will be 

 found to be entirely composed of more or less rounded 



