POTATO AND TURNIP PLANTS COMPARED. 31 



potato, while the drooping and spreading habit of its 

 leaves, gives them a power of evaporation greatly exceed- 

 ing that of the potato. The turnip consequently needs 

 a plentiful supply of moisture in the soil to enable it to 

 keep its more limited powers of absorption always in 

 action, and any surplus moisture it may carry up into 

 its tissues is speedily dissipated by its broad, expanded 

 leaves. We find throughout nature that the various parts 

 of a plant are beautifully proportioned to its natural wants 

 and the duties they have to perform. The roots of the 

 potato, therefore, being naturally intended to occupy 

 and carry on their functions in a comparatively dry soil, 

 are provided with a large amount of absorbing surfaces, 

 in order that they may thus be able to abstract from the 

 soil the moisture necessary for the plant ; while the leaves, 

 from the same cause, having but little surplus moisture to 

 dispose of by evaporation, are placed on an erect-growing 

 stem, and are comparatively small, both in size and in 

 number. If a potato and a turnip were grown together, 

 under the same conditions of soil and climate, it is ex- 

 tremely improbable that they would both thrive equally, 

 their natural requirements being so different. The ab- 

 sence of moisture would allow the numerous absorbents of 

 the potato to exercise their powers, and readily support 

 its vegetation; while the small absorbent power of the 

 turnip would speedily exhibit itself in the stunted develop- 

 ment of the entire plant. Again, if the conditions were 

 changed, the turnip would s^ow, by its vigorous growth, 

 that both its root and leaf powers were being fully and 

 healthily exercised ; while the potato would find that more 

 moisture was absorbed and pumped up by its vigorous 

 roots than its leaves could dispose of, and that its general 

 functions would be impeded, and its healthy development 

 consequently be more or less disturbed. The tissues would 

 become gorged with more matter than they could digest, 



