OTHER WAYS OF PROCURING FOOD 73 



food materials, — carbon dioxide and water — while not always 

 abundant are very widely distributed and in constant motion. 

 Thus animals are, in general, capable of moving from place to 

 place while plants are, for the most part, stationary. 



2. These materials which are used in the construction of food 

 by the green plant come in towards it from all directions, which 

 accounts for the radial symmetry of the plant, with its branches 

 and roots passing out in every direction. 



3. The carbon dioxide and water, while present in sufficient 

 quantities, are not abundant. For this reason it is advantageous 

 to expose as much surface as possible, hence the profuse branching 

 of both stems and roots. 



4. Light is quickly absorbed and penetrates only a short dis- 

 tance. This necessitates structures with a broad surface, which 

 are at the same time thin. Leaves fulfill these requirements ad- 

 mirably and enable much material to be spread out into a thin 

 absorbing surface. 



5. The light and carbon dioxide come from above, while water 

 and mineral salts come from below. This results in the formation 

 of a system for absorbing mineral salts and water from the ground, 

 while an aerial system is also required for receiving light and 

 carbon dioxide. Thus arises the division into root and shoot. 



6. It is advantageous for the leaves to get up into the light as 

 completely as possible. At the same time connection must be 

 maintained with the water supply below, which explains the origin 

 of the various types of stems. 



This is sufficient to show the close connection between the 

 photosynthetic function and the structures of plants as we now 

 find them. There is always the tendency to read into such beauti- 

 ful harmonies purposeful adaptations, but one must ever remem- 

 ber that the successful well-adapted structures that cover the 

 earth at the present time are only a small fraction of the experi- 

 ments of nature. Only living people ever praise the physician. 



QUESTIONS 



1. Some plants have variegated leaves with white margins. How do such 

 white parts get their food? 



2. A noted horticultural firm has offered $1,000 to any gardener who will 

 produce a white-leaved geranium. What are the chances for success? 



3. What are the requisites for a successful graft? 



