igoo-i.] Effects of Water on Foliage Leaves. 241 



form. Plants under experimentation are always regarded as living 

 plants — not simply as so much tissue — and from this point of view the 

 phenomena are chiefly considered. 



In order to clear the way to the main issue it was found necessary 

 to investigate certain questions which at first sight may appear not 

 directly a part of the subject ; but it is only through these subordinate 

 and relative questions that we are able intelligently to explain, with 

 any degree of exactitude, the various elements, as connected with, and 

 forming a part of the whole general subject which is a unit. It is 

 recognized to be of importance that one who undertakes a problem 

 should confine himself strictly to that problem ; but it is of greater 

 importance that one should, while confining himself within the limits of 

 his problem, understand and explain not only the details relative to it, 

 but the relationship of that problem, if solved, to scientific knowledge 

 already obtained. In other words he must be able to assimilate his 

 results with the laws of science. This is mentioned in order that it may 

 be fully understood at the outset that the question has branched out 

 into directions not contemplated at the commencement. 



The "spotting" of the tobacco leaf is dealt with in some detail 

 because of its economic importance, it being a particular aspect of the 

 question involved under the heading : — "The effects of solutions applied 

 to leaf surfaces." 



II. — Historical R£sum£. 



As has just been said in the introduction, the question of water- 

 absorption by leaves, being so important a part of the general subject, 

 and having been dealt with by several writers (some as early as the 

 middle of the 17th century), will have a prominent place in this chapter, 

 not only because of its importance of itself, but also because it is the 

 only side of the problem that has been investigated to any considerable 

 extent, though unfortunately with results that seem never to have 

 settled the question. The subject of the absorption of dilute solutions 

 by leaves, being so closely related to that of water-absorption, would be 

 in a similar position to-day, were it not that attention has been turned 

 in this direction by the practice of spraying plants, subject to fungous 

 diseases, with solutions known as fungicides ; and of killing of weeds by 

 means of a spray of a poisonous solution. 



The first author of any note was Mariotte (1679, p. 133), who 



