FUNGOUS DISEASES. \ 59 



and Missouri are employing the lath screen advantageously in the 

 prevention of a serious blight of this plant, due to a fungus which 

 is believed to gain entrance most readily at the margin of the leaf, 

 possibly following a tendency to sun scald in that area. In opposi- 

 tion to the beneficial effects of half shade, we have also abundant 

 observations showing that certain powdery mildews are far more 

 effective as parasites under just such conditions as above enumerated. 

 I have seen wheat under partial shade badly infested with the 

 powdery mildew, which in the central West, at least, is seldom, if 

 ever, seen in the open. Time and again, in that same region, 

 moreover, I have noticed that in the case of well-watered lawns the 

 mildew of blue grass abounds in a circle rather sharply limited by 

 the heavier shadow areas of trees. Similarly, in that region the 

 grape mildew is, as a rule, found on Vitis vinifera stock only in 

 shaded places. The fungus soon disappears from leaves to which 

 strong sunlight is admitted. Strawberry mildew is also far more 

 abundant in shaded localities. It is claimed that certain greenhouse 

 plants are more subject to the attack of the common gray mold, 

 Botrj'tis, when partially etiolated, and DeBary, it seems, was able 

 to predispose Petunia to the attack of Botrytis through etiolation. 



It is apparent that in so far as screen mechanisms largely prevent 

 the formation of dew, it is probably in large part through this 

 effect upon the moisture relation that they are important. There 

 may also result a number of direct effects of light, for in the case of 

 strongly etiolated or yellowed and attenuated plants, bud and stem 

 diseases seem frequently to be more common. Very little study 

 has been bestowed upon these relations. In this connection, we 

 may briefly indicate some of the modifications in form, size, struc- 

 ture, and chemical content which plants undergo with varying light 

 intensities. Under partial shade accompanied also by certain other 

 changes in the en^^ronment, such as increased or diminished humid- 

 ity and soil moisture, the general effects upon the plants may be 

 remarkable. Plants grown in partial shade show leaves which are 

 larger and thinner; but when still more light is excluded the leaf 

 area will be greatly reduced. Again, under partial shade, the stems 

 are longer and of less diameter. The cell structure is not so com- 

 pact, the cell walls are thinner, the water content is greater, aro- 

 matic substances in the plant are reduced, and the relative acidity 



