GROWTH OF PLANTS. 59 



63. The cellular tissue of plants, while it is still young, and 

 receives a sufficient quantity of nutritious juices, gives rise to 

 new cells, which are at first very small, isolated and soft ; but 

 which, in proportion as they are developed, enlarge and harden, 

 and become as closely united to each other as to the cellular tissue 

 upon the surface of which they are formed. Those cells which 

 have ceased to grow, no longer possess the power of giving rise 

 in this way to new tissue; they become strongly joined to the 

 young cells with which they are in contact; and hence it is that 

 the growth of plants takes place only from the surface of the 

 most recently formed parts. 



64. In ex'ogenous plants, the new tissue is thus deposited 

 between the albur'num and the bark, and at first appears in the 

 form of a viscid matter which is called cam'bium. Those tissues 

 which arise from the albur'num, form around the ligneous body 

 or wood of the stem, a new layer of albur'num, exterior to all 

 those that have been already deposited ; and those which arise 

 from the bark constitute a new cortical layer, within the layers 

 of bark already formed. Each of these layers increases in 

 thickness for a certain time, then ceases to grow, and, at the end 

 of a certain period, in its turn produces a new layer. 



65. Perennial ex'ogenous plants in this way form a new layer 

 of wood and of bark every year; and if we cut through the 

 stem of a tree transversely, we may see the number of zones or 

 rings of which it is composed, and thus count the number of 

 years it has lived. 



66. The thickness of these layers varies in different plants, 

 and also varies in the same tree according to its age, the richness 

 of the soil in which it grows, and the abundance of its leaves, 

 &c. Trees grow most rapidly during the first years of their 

 existence, and it is observed that in old trees the most external 

 ligneous layers are thinnest. When the soil that surrounds the 

 foot of a tree is more favourable to vegetation on one side than 

 on the other, the roots become unequally developed, and on the 

 side where the largest roots are found are also found the largest 

 branches and the thickest ligneous layers. 



67. The new ligneous and cortical layers are not restricted to 

 covering the surface of the plant, but are prolonged beyond it, 

 and, at different points, form lateral expansions which constitute 



63. From what parts does the growth of plants take place ? 



64. What is cam'bium ? How is the new matter deposited ? Do the 

 new layers always continue to grow ? 



65. How long is occupied in the formation of a new layer ? 



66. Is the thickness of these layers the same in all plants ? When is the 

 growth of trees most rapid ? 



67. What are buds? Where are they found? What are the charac- 

 ters of these buds ? Upon what does the rapidity of their growth depend 7 



