LEAF EVOLUTION. 73 



other places plants under a disadvantage, when contrasted 

 with members of the animal kingdom, for the following 

 reason. In order to convert carbonic dioxide, water, and 

 mineral substances anto an organic body, first appearing 

 under the form of starch, the action of chlorophyll working 

 under the influence of light is necessary ; consequently we 

 find that throughout the vegetable kingdom there is a 

 gradual evolution in accordance with this idea, the greater 

 the area of chlorophyll-bearing surface exposed to light the ' 

 better. Thus we get a very large proportion of the substance 

 of the plant flattened out in the form of thin plates, as 

 fronds, leaves, etc., for this purpose, which collectively may 

 be looked upon as a condensing and chemical apparatus for 

 manufacturing protoplasm or organic matter from the raw 

 inorganic material. Subsequently the protoplasm by further 

 chemical changes becomes resolved into starch, cellulose, 

 etc., which corresponds to the starting-point of nutritive 

 processes in the animal kingdom, in the sense of dealing 

 from the first with organic matter. The general habit of 

 plants is also that which favours this process, as shown by 

 the gradual evolution of trunk and branches, which protect 

 the leaves from the attacks of herbivorous animals, and at 

 the same time place them under the most favourable 

 conditions for performing their functions. Finally, we meet 

 in the leaves with a gradual evolution tending towards the 

 same idea, as illustrated by the gradual transition from the 

 entire or undivided form, to the very much cut up or 

 compound leaf, possessed of the power of closing up when 

 unable to perform its functions, and thus more or less effect- 

 ually protecting itself, as in the so-called " sensitive plant," 

 and numerous others. Other modes of leaf protection are 

 met with under the form of spines, bitter tastes, etc. 



