44 GROWTH OF THE PLANT 



weak, brittle stems, where there is insufficient cellulose for 

 the cell walls. Potassium helps the plant to resist attacks 

 of fungous diseases such as the rust. Fungous hyphse can 

 not readily penetrate strong cell walls, (d) It serves also 

 to cause good leaf development, (e) It gives enlarged 

 yields of potatoes, which are a starchy crop. (/) It also 

 increases the yield of sugar in beets (Figs. 8 and 9). 

 This is very well illustrated at the Rothamsted Station, 

 England, where a mangold crop fertilized with nitrogen and 

 phosphorus produced 1594 pounds of sugar, but when fertil- 

 ized with potassium, in addition to nitrogen and phosphorus, 

 produced 4446 pounds, (g) Potassium also is the base which 

 neutralizes or partly neutralizes the acids produced during 

 plant growth. 



43. Nitrogen. — (a) Nitrogen is a necessary constituent 

 of all proteins, and proteins play a very considerable part 

 in the formation of protoplasm, chlorophyl, and many other 

 compounds (Section 33) . (6) It is important in the growth 

 of new tissue, such as stems and leaves (Fig. 10), producing 

 a bright green color in the leaves (effect on chlorophyl). 

 In case an excess is present such a vigorous growth of foliage 

 is produced as to delay maturity of the seed. The ripening 

 of the seed is a process of translocation of food material 

 which does not take place while active assimilation and 

 vegetative growth are going on. This effect of delaying 

 maturity is just the opposite of the effect of phosphorus which 

 hastens it. (c) Excess of nitrogen also causes such rapid 

 vegetative growth that the stems are weakened, and lodging 

 in the case of grain may result. Apparently the cells of the 

 growing stem are multiplied more rapidly than proper cell 

 walls can form, and large cells with weakened walls result. 

 (d) In the case of leafy crops, such as lettuce and cabbage, 

 or of a stem crop like celery, this excess of nitrogen is 

 advantageous. Seeds are not wanted, (e) This effect of 

 too much nitrogen on growth renders plants susceptible to 

 disease. Wheat crops over-stimulated with nitrogen are 

 liable to rust. Greenhouse crops which are always grown 

 on soils rich in nitrogen are peculiarly sensitive to diseases. 

 Shrubbery and young trees which have been forced too 



