THE PLANT BODY 23 



so that a forking appearance is produced in the upper branches 

 and the terminal twigs of the crown. 



In other hardwood trees, like the hard maple and Carolina 

 poplar, false whorls of branches "are formed, resembling very 

 closely those of the pine in distinctness and in the number of 

 branches in a whorl. 



The leaves of the season in the elm and other hardwood trees 

 are disposed as in the pine, at the ends of the terminal twigs 

 of the season. In the spreading type of tree they present an im- 

 mense surface to the sun for photosynthesis, while their spiral 

 or cyclic arrangement secures to them adequate light without 

 the danger of overlapping. 



SUMMARY 



From the above accounts, illustrated by the development of the 

 pine and the elm, it is seen that three main factors determine the ulti- 

 mate forms of trees and the successful display of their foliage, fruits, 

 and seeds. These factors are the body plan, the unequal growth of 

 buds and branches, and pruning effects due to competition in the 

 crown of the tree. Of these factors the unequal growth of the buds 

 is certainly the most important, since by this factor the pyramidal or 

 the spreading form of the tree is determined, as well as the nature 

 and disposition of the false whorls of vigorous lateral branches. 

 In the spreading-tree types the use of the term false whorls is only 

 permissible in order to make clear the close similarity which exists 

 between the mode of annual bud growth in the pyramidal pines and 

 that in the spreading types of deciduous trees. 



In herbaceous plants the same general principles obtain in the 

 development of the mature plant as in trees, and the forms assumed 

 by them correspond, as we have seen, to the erect pyramidal type 

 and the spreading type of the pine and the elm. 



ADJUSTMENTS TO THE ENVIRONMENT BY TROPISMS 



It has just been shown that the inherited plan of the plant 

 body of our common plants is favorable to the proper plac- 

 ing of leaves, roots, and steins for the absorption of food 

 materials and energy from the soil, air, and sunlight. It will 

 be quite evident, however, to the critical observer of plants, 



