286 ]\Ir Philip Scwell on the [sess. lit. 



We may, however, reasonably connect certain well-defined 

 changes which occur naturally, and which are accompanied 

 by the presence of definite substances among the cell-contents, 

 with the naturally present acids or alkalies. The blue, 

 common among the Boragineie, may thus be explained by 

 the well-known alkaline character of the plant. Similarly, 

 the acid property of vine leaves may have something to 

 do with their rich colouring. 



Again, it has been shown that the proportion of various 

 of the substances in the plant may vary at different times — 

 sometimes during a single day. For instance, De Lange has 

 asserted that plants contain a smaller proportion of acids 

 when dead than when alive ; that there is an increase 

 of acidity during the morning, and a decrease at night. 

 Similarly, the proportion of tannin (which often is known 

 to produce a red colour in the cells where it is present) 

 varies in different parts and at different times. It is present 

 in many red flowers, and is in greater quantity in young 

 than in old leaves. 



Such associated sul)stances may commonly be oidy pro- 

 duced as results of the same metabolic changes which 

 produce colour ; it cannot be denied that they must also 

 influence the colour more or less conspicuously. 



2. Passing on to consider those transitions which are 

 due to the direct or indirect eifects of environment, it is 

 necessary to notice that this part of the subject has, 

 especially in recent years, received a considerable amount 

 of attention from Continental and English observers, various 

 researches having been made as to the iiilluence of light, 

 heat, moisture, soils, or (comprising all these), as to effects 

 of climatal roTiditious on vegetation generally, or on colour 

 particularly. 



(A) To confine our attention to the first of these, we will 

 consider the eHects of light as they are (a) direct, or (|8) 

 indiiect. 



(a) We have before alluded to tlu; iiiii)ortance of light 

 in effecting the change from etiolin to chlorophyll; it is 

 necessary to add that a difference in colour of foliage may 

 be noticed according to the amount of exposure, within 

 certain limits, for instance, where leaves have been less 



