GENERAL CONSIDERATION OF PLANT DISEASES 277 



and after a few hours the formation of starch in the chloroplasts will 

 be detected.^ The storage of reserve materials is, therefore, inhibited, 

 and one finds in such plants, as the cereals, that the formation of green 

 parts is at the expense of the grain, and in the beet, the vegetative part of 

 the plant is at the expense of the fleshy roots. Potassium hunger causes" 

 in the potato and buckwheat a shortening of the internodes and a 

 convex bending of the leaf blades, which are spotted with yellow blotches. 

 Calcium is abundant in nature in the form of the carbonate which 

 forms the rocks known as marble and limestone. It is chiefly concerned 

 in the strengthening of the cell wall, where in such plants as Chara it is 

 deposited. It plays an important role in fixing the calcium oxalate 

 formed in the metabolism of the plant. Ecologists in Europe classify 

 many plants either as calciphile (calcium-requiring), or calciphobe 

 (calcium-hating). The apphcation of calcium to soils under certain 

 conditions promotes apparently the disease of beets called heart- or 

 dry rot. The chlorosis, or icterus, of the grape vine seems to be in- 

 creased in soils with a high calcium content. The accumulation of 

 oxalic acid in the absence of its fixation by calcium poisons the plant. 

 The formation of brown blotches on leaves, the yellowing, or brown- 

 ing of pine needles, the death of the root tips of water plants is associated 

 with the absence of calcium. 



Magnesium is chemically allied to calcium, but it cannot replace 

 calcium in the economy of the plant. It apparently works together 

 with nitrogen in the formation of protoplasm, and has an influence in 

 the formation of chlorophyll, for plants grown without magnesium have 

 yellowish-green chloroplasts, and new cell formation does not proceed 

 readily. The absence of magnesium is shown in the pale-green color 

 of the chloroplasts, the yellow to orange-yellow blotches on the leaves, 

 and the brown spots on the stems. The amount of starch formed by 

 the chloroplasts is reduced, the internodes are shortened, the young 

 leaves do not unfold. These are symptoms associated with a 

 deprivation of magnesium. 



Iron is necessary in the formation of chlorophyll, for if the plant is 

 grown in an iron-free solution, it remains permanently etiolated 

 (blanched). The diseased condition which arises through the lack of 

 the requisite amount of iron is called chlorosis. Too much iron in the 

 soil acts poisonously. 



^Hartwell, B. L.: Bull. 165, R. 1. Agric. Exper. Stat., May, 1916. 



