106 PLANT PHYSIOLOGY 



ment of the vascular tissue leading to them. The shoots become 

 very brittle (Johnston), the phloem of the petioles becomes dis- 

 organized, with the result that the sugars are not transported out 

 of the leaves, and the plants (tomatoes) then develop a purplish 

 color, probably due to the development of anthocyan (Chap. XV). 



Manganese. —This element is another of the newcomers among 

 the list of essential elements, but sufficient work has been done 

 upon it the past ten years to convince all but the most conserva- 

 tive physiologists that it is really a necessary element for the 

 growth of plants. It is found in all plants and in all regions, but 

 especially in those portions which are physiologically active (seeds, 

 root and stem tips, etc.). For this reason and the fact that it is 

 needed in such minute quantities, it is thought by many to func- 

 tion somewhat like a vitamin (Chap. XVII). Others, however, 

 consider that it functions more as an enzyme or catalyzer (Chap. 

 XVI). In its absence, chlorophyll does not develop properly, and 

 the plants become chlorotic. It seems to favor oxidation processes 

 in the higher plants and fermentation in yeast. Its complete ab- 

 sence from oats causes "grey speck," but so little as 1 part in 

 4,000,000 will produce recovery or prevent the appearance of the 

 disease. Above 1 p. p. m. manganese tends to become toxic. It is 

 thus seen to be needed in extremely minute amounts, and ac- 

 cording to McHargue plants growing in unglazed earthen pots 

 may get all the manganese they need from that which dissolves 

 out from the vessels into the water. Although Clark and Ely 

 (1930) did not find this element necessary for Lemna, McHargue 

 and Calfee (1932) found that it was. The place of this substance 

 among the "big twelve" commences to be so firmly established 

 that the burden of proof now rests upon those who wish to prove 

 its unimportance. 



Other Common Elements. — Although the eight elements men- 

 tioned are the most important ones for green plants, certain other 

 elements are of great importance in many cases. Because an ele- 

 ment is not essential, it does not follow that it is not beneficial. 

 Probably most ash constituents exert either a harmful or bene- 

 ficial effect upon the plant which contains them. 



Aluminum. — Aluminum is very common in plant ash. In the 

 case of the common hydrangea the color of the flowers is affected 

 by the presence of aluminum. It had been observed by gardeners 

 that red-flowered forms when grown on certain soils produced 



