14 BOTANY OF THE LIVING PLANT 



zones may be found successively lower down, marking the limits 

 of the increment of growth of earlier years. Each year's growth 

 leaves its record on the outer surface of the branch. Thus, passing 

 from below upwards along the twig, its annual history can be read, 

 till we arrive finally at the terminal bud, which is already providing 

 for the development of the next year's shoot. 



The most important factor in determining the conformation of the 

 plant-body in all the Higher Plants is the continued growth at the 

 apex of stem and root. The life of the Higher Plants may be described 

 as an indefinitely continued embryology, while the provision for increase 

 in number of parts is in a geometrical ratio. In this it differs essentially 

 from that of the Higher Animals, in which the parts of the body are 

 laid down once for all in the initial steps of development, and the body 

 is of a circumscribed and limited type. 



But though the body of the Plant is thus theoretically unlimited 

 in its plan, in actual practice limits are imposed. It would be a 

 physical impossibility to develop all the potential parts of so complex 

 a system. Many buds remain dormant. In others seasonal con- 

 ditions may check or stop apical growth. Various mechanical or 

 physiological injuries may intervene, caused it may be by wind or 

 frost. Animal or fungal attack may destroy many embryonic buds. 

 But probably the most potent check of all is the physiological drain 

 of flowering. This is particularly effective in the case of herbs, and 

 especially of annuals, as in the Sunflower or Bean. By such influences 

 the theoretically unlimited plan of development is restricted within 

 bounds. 



When speaking thus of a theoretically unlimited plan of development it is 

 necessary to make a reservation. The plan itself is one which has no limit ; 

 but in practice an extreme limit is imposed by the principle of similar structures. 

 In similar structures varying in size the strength increases as the square 

 of the dimensions, while the weight increases in the higher ratio of the cube. 

 For every structure, whether living or not, there will accordingly be a limit 

 of size beyond which that structure cannot be enlarged, and still maintain 

 its form, unless there be either a change of plan, or a difference of material. 

 This principle recognised by Galileo has been applied to animals and plants, 

 and explains many peculiarities of large organisms. The greatest height 

 has been calculated to which a tree of given proportions and materials can 

 grow, and still hold itself erect. Some of the tallest trees have approached 

 that limit, which is somewhere beyond 300 feet. Examples of this are 

 seen in the giant trees of California (Frontispiece). The expressions used in 

 the preceding paragraphs naturally apply to organisms of moderate dimensions, 

 and especially to the plan of their construction. 



It is upon a scheme such as that laid down in the preceding 



