192 TEXTBOOK OF BOTANY 



Interpretations exemplified. The reader may be satisfied with the in- 

 ferences he drew and the interpretations he made as he read each of the 

 above paragraphs; or he may wish some means of checking them. Only 

 one example will be discussed here, namely, the relative amomit of 

 growth of roots and shoots of plants supplied with different amounts of 

 nitrates in the soil. If the supply of nitrates is very low the root system 

 is large in proportion to the tops. If it is verv high the root system is 

 small in proportion to the tops. One might speedily dismiss the whole 

 matter by saying that in the first instance the roots grew long in search 

 of more nitrates for the tops, and in the second instance the tops had 

 plenty of nitrates and the roots did not have to grow long in search for 

 more. But would one be content to ascribe to roots either the intelli- 

 gence necessary to diagnose the needs of the tops, or the superhuman 

 ability to grow, or not to grow, at will? 



Now, if we try to explain these growth phenomena on the basis of the 

 manufacture and uses of food in plants, it is necessary only to recall 

 and relate those groups of facts which showed that ( 1 ) foods are used in 

 the building of cells and that growth does not occur without them; (2) 

 the amount of sugar that enters the non-green parts of a plant depends 

 upon the extent to which photosynthesis exceeds respiration and other 

 uses of sugar in the chlorenchyma; (3) no part of the plant can grow 

 well unless it is adequatelv supplied with both sugar and proteins, or 

 with the conditions necessary for making them; (4) although rapid 

 growth depends upon a plentiful supply of proteins from which the new 

 protoplasm is largely made, considerably more sugar than protein is 

 consumed in the building of new cells; (5) particular parts of the plant 

 are dependent upon the movement of certain materials from other parts; 

 and ( 6 ) a definite set of conditions is necessary for the manufacture of 

 sugar and proteins in plants. Fats were omitted from this summary be- 

 cause they seem to be made readily enough in the cells when sugar is 

 present. 



Since all the sugar is made in the tops of the plant, the roots will get 

 an ample supply of it only when it is made in the shoots much more 

 rapidly than it is used there. On the other hand, the shoots will have a 

 good supply of the nitrates needed in protein synthesis only when the 

 amount that passes into the roots from the soil greatly exceeds the use of 

 nitrates in protein synthesis in the roots. 



Chemical analyses of plants show that if the supply of nitrates in the 

 soil is low the tops have larger amounts of carbohydrates in proportion 



