30 THE POTATO CROP. 



immediately beneath we have the stolons and underground 

 shoots developed into irregular shapes and sizes, forming 

 what we call the " tubers/' or more commonly the pota- 

 toes, these again being terminated by the true roots of the 

 plant, upon which alone it has to rely for its supplies of 

 food materials from the soil. These roots are very nume- 

 rous, being attached to each of the stolons or underground 

 stems, and are all provided with a very large number of 

 "spongioles" or absorbent points, by which alone the 

 moisture and food materials necessary for its healthy de- 

 velopment can be assimilated and carried up into the circu- 

 lation of the plant. Passing upwards into the stem and 

 leaves, these food materials come under new influences, 

 and are elaborated into vegetable tissues and other different 

 forms of matter, the surplus moisture being got rid of by 

 evaporation from the leaves. We might reasonably de- 

 duce from these observations that the potato, owing to its 

 large root-development and comparatively small evapora- 

 tive surface, is a plant essentially suited to a dry soil, and 

 that any departure from the conditions natural to it must 

 be followed by consequences more or less injurious to its 

 general health. These, as we have just observed, would 

 be greatly affected by the climate of the district ; if it be 

 moist, the evaporative power of the leaves would be 

 lessened; if it be dry, they would be increased, so as pro- 

 bably materially to counteract the effects of surplus mois- 

 ture in the soil. 



If we compare the structural arrangement and con- 

 sequent functions of this plant with those of our ordi- 

 nary root plants take the turnip for instance we find 

 a vast and important difference to exist. The turnip 

 sends down into the soil a single tap-root, from which 

 depart numerous rootlets, each terminated by a small 

 feeding point; but these are very much smaller in 

 number, and less in absorbing power than those of the 



