229 



The red spot disease of the leaves, occurring to a devastating extent 

 along the v^-hole coast, was a second phenomenon. A pale border appeared 

 along the edge of the leaves ; the zone was distinctly cut off from the inner 

 portions by a zigzag line. Dark red spots, or a uniform red coloration with 

 which a deforming of the leaf surface was often connected, then appeared. 

 The disappearance of this trouble with the appearance of drought indicates 

 that the soil during the prevailing wet weather had .unfavorably affected the 

 growth of the cotton. Vosseler seems to suspect that the dreaded "wilt dis- 

 ease" should be included among the climatic diseases and refers in this to 

 the possibility -of producing immune races by growing plants from seed of 

 healthy stock in diseased fields. According to Schellmann^, cotton cannot 

 grow on stiff clay soils and sour humus soils. 



Castor Bean Cultures. 



Although Ricinus thrives in subtropical and even in temperate zones, 

 according to Zimmermann'-', it is extensively cultivated only in the tropics 

 where it grows from sea level up to possibly 1600 m. The oily seeds are the 

 desired crop. At any rate an abundant supply of nutriment is needed for 

 Ricinus, since it makes very great demands on the soil. The plant also re- 

 quires large amounts of water while growing. Later, however, the physical 

 constitution of the soil has a determining value in the matter, since the plants 

 do not thrive in all soils which, not well drained, remain constantly 

 wet. These observations in the tropics correspond with our experience in 

 growing Ricinus as a decorative plant. Only the plants develop well which 

 have plenty of room and a porous soil, rich in nutriment. When grown in 

 pots, to which much nutriment is added by fertilizing salts, the earth becomes 

 encrusted and the plants remain small and weak. 



Tobacco. 



Very instructive examples of the determinative influence of the soil are 

 furnished by Hunger's'* observations on the development of the Delhi- 

 tobacco and its different behavior toward the "Mosaic Disease," which will 

 be reported more fully in the section on enzymatic diseases. 



Hunger says that a soil of white clay in which much sand has been 

 mixed, is the best for thin-leaved tobacco if the amount of precipitation is 

 favorable, but at the same time this also favors most the abundant appear- 

 ance of the mosaic disease in the form of the so-called "yay-head." Here., 

 after topping, the plant gives the impression of having made too rapid 

 growth; long internodes, a yellowish-green foliage, a great many lateral 

 shoots, all of which are sickly. 



1 Der Pflanzer, Usambara-Post, 1905, No. 1. Here also older literature. 



2 Zimmerman, A., Die Ricinus-Kultur. Der Pflanzer, Ratgeber fiir tropische 

 Landwirtschaft unter Mitwirkung- des Biologisch-Landwirtsch. Institutes Amani, 

 herausg. durch d. Usambara-Post 



?' Zeitschr. f. Pflanzenliranlvh. 1905, Part 5. Hunger, as Botanist at the experi- 

 mental station for Delhi-Tobacco (VIII Abt. d. Bot. Gart. zu Buitenzorg) has had at 

 his disposal most extensive material for observation. 



