114 Introduction to Botany. 



88. Influence of Light and Gravity. — While light and 

 gravity are not necessary to the immediate processes of 

 growth, they do influence the direction and character of 

 growth of the plant members, as we have already seen. 

 Light has much to do with the size and form of the mem- 

 bers. Compare, for instance, a shoot of Virginia creeper 

 which has grown in the dark with one which has grown 

 fully exposed to the light (Fig. 48). We see in such a case 

 that the internodes of the shoot which has grown in the 

 dark are much longer than those of the shoot which has 

 developed in the light, while its leaves are considerably 

 smaller than those of the illuminated shoot. We can see 

 the usefulness of the habit of greatly elongating inter- 

 nodes and keeping the leaves reduced where the shoot is 

 in much darkened places, for where the leaves cannot 

 have access to the light they require materials for their 

 production which they cannot replace by the manufacture 

 of food materials. The greater elongation of internodes 

 in the dark brings the leaves more certainly and quickly 

 into illuminated places. 



89. Rings of Annual Growth. — When a perennial di- 

 cotyledonous plant resumes growth in the spring, it pro- 

 duces more branches and leaves than it possessed the pre- 

 vious year. This is characteristic of branching perennial 

 plants. In consequence of the increased transpiring sur- 

 face and weight of the crown, more demands are made on 

 the stems and roots, for they must be stronger, and they 

 must be able to conduct larger supplies of water to the 

 leaves. Here is where the usefulness of the cambium 

 ring is shown; for while the leaves and branches are being 

 formed, the cells of the cambium ring are dividing and 

 adding to the thickness of the stems and roots. The tis- 

 sues which are first formed by the cambium are of a nature 



