366 The Living Plant 



includes everything outside of the cambium. Though the bark, 

 like the wood, thus increases in thickness as long as the tree lives, 

 at least theoretically, in practice it weathers away about as fast 

 on the surface as it forms inside. It will now be clear why this 

 exogenous mode of growth permits the indefinite expansion of 

 woody stems. 



In close relation to the age at which flower buds first appear is 

 the length of life of the plant. When plants come to flower the 

 season they germinate, all of the food they can make is thrown 

 into their seeds, and the soft stems then die, root and branch: 

 such plants are annuals. Other kinds, however, make only leaf 

 buds the first season, and store up food in some underground part 

 to which they die down; then this food is made use of in forming 

 new stems, flowers and seeds the next season, after which the 

 plants perish completely: such plants are biennials. Yet other 

 kinds, in the second season, instead of throwing all of their food 

 into seeds, store a part underground, die down thereto and then 

 send up a new flowering stem the next season, and so on year after 

 year: such plants are herbaceous perennials, which include so 

 many of the favorites of our gardens. Finally, there are many 

 others which do not die down to the ground at all, but harden 

 their stems to wood, and thus can stand upright over winter. 

 Thereafter, each season's growth, whether in length or in thick- 

 ness, is built upon that of the preceding, and the structure thus 

 grows both hi length and in thickness as long as it lives: such 

 plants are woody perennials, which are principally shrubs and 

 trees. Since this method admits of indefinite increase in size, and 

 since, moreover, it involves a constant rejuvenescence of the 

 protoplasm (the significance of which is discussed earlier, on 

 page 162), it is obvious that trees have no limit set to their growth 

 by internal factors, but their maximum size is imposed by the 

 action of extrinsic causes, such for example as the increasing 

 difficulty of conducting sufficient water supply through the 

 greatly lengthening stems. Thus, with increasing size it becomes 



