56 TEXTBOOK OF BOTANY 



chapters. At present it is more important to know that bright Hght often 

 checks the rate of growth in length of plant stems, and to understand 

 how this fact may be used to explain why stems and petioles bend when 

 the light to which they are exposed is not uniform on all sides. 



The question of whether the result is good for the plant or plant part 

 does not arise in this type of explanation. Whether it be advantageous 

 or disadvantageous is another matter entirely unrelated to the cause of 

 the behavior of the plant. Results follow causes regardless of whether 

 they are good or bad for the organism. In fact, the result often is the 

 death of the plant. 



The generalization that bright light has an inhibiting effect upon 

 growth in length of plant stems is true in numerous instances, but one 

 should not immediately infer that stems always grow more rapidly in 

 length in the shade than they do in bright light. The examples cited in 

 the discussion above were seedlings and sprouts. On the other hand, 

 young trees in heavily shaded forests grow in height very slowly for 

 many years. If they become exposed to increased light through the death 

 or removal of an older tree, their rate of growth in height increases 

 (Fig. 29). The differences in rates of growth of these young trees are 

 consequences of certain processes which the reader will appreciate when 

 he has become familiar with the facts in the chapters on plant nutrition 

 and water relations. They in no way contradict the explanation of the 

 example cited above, but they do emphasize the fact that cell division 

 and cell enlargement are influenced by many conditions within the cells, 

 and that under other circumstances another group of internal conditions 

 may become the important one. 



Interpretation of hereditary differences. The previous paragraphs in 

 this chapter have centered on differences in plant behavior which are 

 due to a lack of uniformity in the environment. In Chapter I attention 

 was called to other differences, such as the presence of leaves, roots, and 

 seeds in some plants and their absence from others in all environments. 

 Many writers seem to think that such structures developed to meet a 

 need. The student of plant processes, however, is more likely to regard 

 the origin of these structures as consequences of changes that occurred 

 within the cells of remote ancestors without reference to the needs of 

 the plants of today. To him they are the result of heritable alterations in 

 the chemical and physical composition of the protoplasm. Purpose can 

 scarcely be thought of as the cause of these minute changes in the com- 

 position of the protoplasm, or of the far more striking differences in struc- 



