i 4 BOTANY OF THE LIVING PLANT 



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. 



Any Land Plant built upon such a progressive scheme as this 

 requires as it grows additional provision for mechanical support, and 

 for the conduction of water and other supplies from the soil. This is 

 achieved in various ways, as will be seen later. But the most prevalent 

 is the method seen in forest trees, in which the trunk and branches 

 thicken according to the demands of the enlarging body. It is common 

 knowledge how the wood of their trunks is marked internally by 

 annual rings, so called because normally one ring of new tissue is 

 added each year. A similar growth is seen in their roots, both regions 

 showing an automatic increase to meet the growing demands, which 

 are both mechanical and physiological. 



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, the increase in the number of 

 parts being in a geometric 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. 

 Consideration must also be given to the physiological drain of 

 flowering. This appears to be 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. 



It is upon the scheme laid down in the preceding pages that the 

 body of all the Higher Plants has been constructed. The number 

 and exact position of the leaves may vary, and consequently the 

 number and position of the branches, since these arise from axillary 

 buds. The form and proportion also of the axes and leaves is open 

 to great difference of detail ; they are frequently adapted biologically 

 to the conditions under which they live. But these are only minor 



