CONSTRUCTION OF THE SHOOT SYSTEM 51 



rule for the other. There is nevertheless a general 

 agreement between the two, though careful observa- 

 tion shows that similarity of arrangement subserves 

 very different purposes. The arrangements of the shoot 

 all bear a certain relationship to life in air and its conse- 

 quent requirements, and show further a co-ordination 

 with the needs which are cared for by the roots. 



We have seen that one of the primary objects of the 

 latter is to secure a firm anchorage for the plant that it 

 may be able to maintain its erect position. The de- 

 velopment of a large head or upper part makes against 

 such anchorage, by offering a large area to the pressure 

 of wind and the beating of rain forces likely to lead 

 to uprooting from the soil. 



We may ask why such a risk should be undertaken 

 why the sub-aerial portion of the plant need attain the 

 large dimensions it possesses. What are the advantages 

 which are afforded by a widespreading head rearing 

 itself into the air? Are they commensurate with the 

 risk, and what are the precautions which protect the 

 plant in face of the dangers it involves ? 



In seeking answers to these questions we must look a 

 little more closely at the chief features of the upper 

 portions of the shoot system. We soon see that one of 

 the objects secured by the method of development which 

 it follows is the great amount of surface in proportion to 

 bulk which the shoot presents. The twigs are thin, the 

 leaves flat. We have indeed, as we have in the root, 

 and as we notice in the case of the large seaweeds, the 

 bringing of the structure of the plant into relationship 

 with as large a portion of the environment as possible. 

 Here is clearly an indication or suggestion of an inter- 

 change of material between the two. 



We have already assumed that there is such an inter- 

 change, and may now examine more closely its nature. 

 A few simple observations will enable us to prove it. 



