540 



much or all of the light falling on a leaf surface, hence photo- 

 synthesis theoretically must cease and starvation of plant tissues 

 result. 



However, we know that for many years fruit growers have 

 sprayed their orchard trees with insecticides and fungicides 

 containing lime, and that the coating left on the foliage some- 

 times for weeks appears to work no injury whatever. Instead, 

 the foliage seems more healthy and vigorous. This lime coating 

 is not black and opacjue, but it is semi-opaque and excludes a 

 considerable part of the illumination falling upon it. 



The question naturally arises,- then, as to the actual effect 

 upon photosynthesis of a leaf coating of any sort or color. 

 Does a coating or crust of any material on a leaf surface func- 

 tion to reduce or retard photosynthesis as does an opaque 

 screen when interposed between leaf and light source? 



When a green plant is surrounded by a screen which 

 excludes all light but admits air, the reserve starch in the 

 leaves is used up and none is elaborated to replace it. An 

 examination after several hours shows an absence of starch. 



During the night most plants use up the excess starch stored 

 in the leaves during the day, so that an examination at dawn 

 will normally show an absence of that carbohydrate. Such is 

 the case with orange leaves. 



Xow, if a leaf coating of honey-dew smut or of carbon- 

 black or of any other substance acts like an opaque screen to 

 reduce or inhibit photosynthesis, we should expect to find a 

 shortage or absence of starch in the leaf beneath the coating. 



There are two means of determining the presence or absence 

 of carbohydrates in green leaves. One of these, known as 

 Sach's ^lethod, is comparing dry weights of ec|ual areas of 

 leaf tissues. The other is the familiar iodine test for starch. 

 The first tests for all carbohydrates, the second for starch only. 

 In connection with protracted investigation carried on by 

 the writer in California upon the subject of alleged injury to 

 orange trees by deposition of cement dust on the foliage, some 

 important data is available on the larger subject which forms 

 the basis of this paper. 



