24 DISEASE IN PLANTS. 



the conditions of light and moisture, and this 

 affects the whole mechanism of transpiration 

 especially, and therefore the supplies of water 

 and mineral salts. Nevertheless, some interesting 

 and valuable results have been obtained in con- 

 nection with this important subject. 



It was found, for instance, that the foliage of 

 a sun-flower or of a vegetable-marrow may be 

 forming starch at a rate of considerably over a 

 gram per hour in every square meter of leaf- 

 surface exposed on a fine day; while in particu- 

 larly clear and warm sunny weather Sachs 

 obtained as much as 24 to 25 grams per square 

 meter per diem. 



When one reflects that 200 square meters is 

 not an extravagant estimate for the area of leaf- 

 surface exposed on a tree, for a period which even 

 in our latitudes may be considerably over 100 

 days of, say, ten hours' light, we need no longer 

 wonder at the rapidity with which wood is pro- 

 duced in the stems, and similar estimates (which 

 I have purposely kept lower than the estimates 

 for continental and tropical climates) may suffice 

 to show how quickly potatoes or the ears of corn, 

 etc., may fill up with the starch or other carbo- 

 hydrates which render them valuable as crops. 

 We want more measurements in these connections, 

 moreover, for there are several ways in which 

 they are of scientific value and practical import- 

 ance. 



It is evident from what has been said that 

 every grain of starch formed represents so much 



