^ LESSONS m CHEMISTRY. 



production of new layers external to those previously formed. 

 In the other division they have arranged all those plants in 

 which the growth takes place by the formation of new wood 

 at the centre.* If you cut a stalk of young grass across you 

 will have an opportunity of examining the structure of a plant 

 of the second of these divisions. When it is viewed with the 

 microscope, it appears to consist of a mass of cellular fibre, 

 like the pith through which a number of tubes extend in a 

 vertical direction. 



88. For the sake of clearness, we will, however, confine 

 our attention to the structure of the plants of the first division, 

 which includes the greater number of forest-trees, and among 

 our crops the potato, the turnip, the carrot, the bean,' and 

 the pea. 



89. The branches are simply prolongations of the stem, and 

 like it, consist of bark, pith, and woody matter. 



90. The leaves exhibit almost every variety of form and 

 beauty, and are of the greatest importance to the gi'owth of 

 plants. They consist internally of a fine network of branching 

 vessels, which may be regarded as extensions of the vertical 

 tubes of which the wood of the stem is composed, while the 

 green exterior part is traversed by minute vessels which spread 

 themselves on the surface of the leaf, and communicate with 

 the vessels which run along the inner layers of the bark. The 

 entire leaf is covered with a delicate membrane, which is an 

 extension of the epidermis, and is perforated with minute 

 holes or pores.f These little openings are especially numerous 

 on the side of the leaf which is turned towards the ground. 



91. The root, like the branches, is considered to be merely 

 a continuation of the stem of the plant, and in its structure 

 there is considerable resemblance; but as it descends into 

 the earth its texture alters very much, it gi'ows soft and 

 spongy, and loses the green colour which is displayed by the 

 parts above the surface. It sends off into the soil in all 

 directions minute hairhke branches^ which the microscope 



* Such plants are termed endogenous from growing at the centre, and 

 have but one lobe in the seed, while in the exogenous plants, as in the 

 common bean, the seed is capable of being divided into two or more 

 lobes ; the grasses, among which botanists include wheat, barley, oats, 

 rye, rice, &c., belong to the endogenous plants. 



f Stomates. % Radicles. 



