xxi] ROOTED AND ROOTLESS PLANTS 265 



the same subject, of which the interpretation is a less simple 

 matter. He made comparative cultures of certain submerged 

 species (Vallisneria, Elodea, etc.) rooted in soil, rooted in washed 

 gravel, or anchored above the soil in such a way that the roots 

 were unable to penetrate it. He found, throughout, that the 

 rooted plants grew much better than those that were merely 

 anchored. Very similar results have been obtained more re- 

 cently by a German botanist 1 , whose experiments may be illus- 

 trated by means of a single example. A number of shoots of 

 Elodea canadensis were planted under water in soil in which they 

 were allowed to take root. Another set of shoots, equal in 

 number and approximately equal in size, were placed in the 

 same glass receptacle, but were supported above the bottom in 

 such a way that their roots were unable to penetrate the soil. 

 After 2 8 days the experiment was interrupted, and the two sets 

 of shoots were measured. It was found that the rooted shoots 

 had grown much more rapidly, their total length amounting to 

 308-0 cms., as compared with 177-5 cms. in the case of those 

 which had been prevented from taking root in the soil. The 

 interpretation of these and similar results has been the subject 

 of some controversy. Pond deduced that the primary cause of 

 the retarded growth of the non-rooted plants was their inability 

 to secure enough phosphorus and potassium and possibly 

 other elements. He found that such plants, in the case of 

 Vallisneria^ were not only stunted in growth, but had their tissues 

 loaded with an abnormal amount of starch; he came to the 

 conclusion that lack of certain salts inhibited proteid synthesis 

 and growth, though the conditions were favourable to photo- 

 synthesis. Another American author 2 has recently published 

 results, bearing on this question, which it seems impossible to 

 reconcile with the views of Pond. He finds that the difference 

 in growth between rooted and unattached plants can be alto- 

 gether eliminated by passing carbon dioxide through the water 

 several times a day. He considers that the non-rooted plants 

 do not suffer at all from lack of salts, but chiefly from lack of 

 1 Snell, K. (1908). 2 Brown, W. H. (1913). 



