452 NITROGEN jVIETABOLISM 



cells by any meristem therefore requires a supply of both carbohydrate and 

 proteinaceous foods (largely amino acids). Considerable quantities of carbo- 

 hydrates are also consumed in any actively growing meristem in the process 

 of respiration. 



With the sole exception of water it is these two types of compounds which 

 are used in the greatest quantities by meristems in the construction of new 

 cells. If the supply of amino acids, etc. to any growing meristem is abundant 

 relative to the supply of carbohydrates a large quantity of protoplasm will be 

 synthesized relative to the amount of cell wall material formed. The resulting 

 cells will usually be large, thin-walled, and well stocked with protoplasm. 

 Tissues composed largely or entirely of such cells are usually soft and succu- 

 lent and contain very little mechanical tissue. 



If the converse situation prevails, and carbohydrate foods are relatively 

 more abundant than organic nitrogenous compounds, proportionately more 

 cell wall material and less protoplasm will be synthesized. The resulting cells 

 will be small, thick-walled, and contain only relatively small quantities of 

 protoplasm. Tissues composed largely or entirely of such cells are usually 

 compact and more or less woody. The development of fibers or other me- 

 chanical tissues is favored by nitrogen deficiency, as is also the formation of 

 cutin on leaves. 



Since both types of compounds are required for normal vegetative growth, 

 extreme deficiency of either organic nitrogenous compounds or of carbohy- 

 drates results in a stunting of plants. Dearth of the former limits growth 

 because little or no new protoplasm can be synthesized ; dearth of latter 

 largely or entirely checks synthesis of new cell wall material and also checks 

 formation of protoplasm because carbohydrates are required in amino acid 

 synthesis. The plants in class IV of Kraus and Kraybill's classification are 

 clearly nitrogen deficient plants {cf. Table 41). Correspondingly those of 

 class I are obviously carbohydrate deficient plants. 



The type of vegetative development exhibited by the plants of class II 

 is characteristic of those receiving an excess of nitrogen, but which nevertheless 

 contain a sufficient supply of carbohydrates to permit the maintenance of vege- 

 tative growth. In such plants a large proportion of the carbohydrates syn- 

 thesized are converted into amino acids which favors the development of large, 

 thin-walled cells, and a generally rank or succulent growth habit. The vege- 

 tative responses of the plants in class III are characteristic of those in which 

 the proportion of carbohydrates to organic nitrogenous compounds is some- 

 what greater than in class II. Cells in general are smaller, their walls are 

 thicker, and the tissues in general are relatively firm although not excessively 

 woody. 



