STRUCTURE AND GROWTH OF PLANTS. 65 



j;reen leaves are hung around it upon numerous branches as 

 if to seize the winds that pass over them. 



85. The stem. — When the trunk of a tree which has been 

 cut across by the carpenter, is examined, it is found to consist 

 of a number of layers or rings embracing each other, and en- 

 closing a central mass termed the pith. The hark forais the 

 exterior portion of the stem, and is capable of being divided 

 into several distinct layers, the outer of these layers which 

 has been compared to the thm membrane (cuticle) that covers 

 the human body, and which rises into a bladder when the skin 

 is blistered, is termed the epidermis. In plants having hollow 

 stems like the grasses, the epidermis is a part of great impor- 

 tance, and is found to contain a large amount of silica, (50) 

 forming a glassy network which gives strength to their struc- 

 ture. In some plants there is so much siliceous matter de- 

 posited in the epidermis, that when rubbed together they pro- 

 duce sparks. 



SQ. Within the layers which form the bark, we observe a 

 series of rings composing the wood^ the outer of these layers 

 are soft and spongy, and are the latest formed portions of the 

 stem. When the wood and inner layers of the bark are ex- 

 amined by a microscope they are observed to be composed of 

 hollow tubes or vessels, which extend from the root to the 

 branches, while the central spongy mass, the pith, consists 

 chiefly of cellular fibre, (60) traversed by tubes which are 

 arranged in a horizontal direction. In old forest-trees the 

 pith is found to have entirely disappeared, and to be replaced 

 by firm wood, and in many of our rapidly growing cultivated 

 crops, as in the carrot and parsnip, it is torn up by the gi'owth 

 of the plant, leaving a hollow stem. 



87. Though the stems of the greater number of trees 

 possess the regularity of structure just described, yet many of 

 our most familiar plants, as the grasses, exhibit an entirely 

 'lifferent arrangement of parts. This difference of structure 

 iias led Botanists to divide our cultivated plants and forest- 

 trees into two gi-eat classes, which they have designated by 

 terms derived from the Greek language, one of those classes 

 comprehends all those plants in which, as in that lately de- 

 scribed, (85) the exterior layers of the woody matter of the stem 

 are the latest formed,* the plant growing, as it were, by the 



• The plants of this dirision are termed exogenous, from growing at 

 the outside. 



