BOOKS AND CURRENT LITERATURE 363 



and the shoots of the plants were subject to the temperature changes 

 incident to the experimental garden at Carmel. As is well known 0. 

 versicolor is native to southern Arizona and thus, in its proper habitat, 

 is exposed to a warm summer climate. At Carmel, however, the tem- 

 perature of the soil throughout the year is relatively low and so is that 

 of the air. It was not especially surprising, therefore, to learn that 

 shoot growth of the species scarcely ever occurs at the Coastal Labora- 

 tory. This being the case experiments were planned as suggested 

 above, and the apparatus was arranged in the following manner. A 

 root box was devised and constructed in such a manner that heat could 

 be applied continuously at each end by using electric lamps. A part 

 of one side was made of glass and sloped in such a way as would bring 

 about the exposure of the roots. The entire box was well insulated, and 

 the soil was closety covered with heayv' building paper. The tempera- 

 ture of the soil was slightly above the optimum for the Opuntia at the 

 ends of the box, and somewhat under the optimum at the middle, but 

 between the two the temperature was, for the most part, close to 

 30°C. Several specim.ens were removed from the garden, where they 

 had been quiescent for the entire season, and put in the box in early 

 September. The culture was left under a protecting shade until late 

 November during all of which time the soil was frequently watered 

 and was kept heated in the manner suggested, and the shoots of the 

 plants were exposed to uncontrolled air temperatures, which varied 

 between 3.5° and 30°C. as extremes. 



Such being the conditions of the experiment, the results can be stated 

 in a word. Root growth of all plants in the soil where the temperature 

 was most favorable was found, when the experiment was terminated, to 

 have been very active, and the roots reached to the bottom of the box, 

 or to a depth of 60 cm. All of the plants were found to have put on new 

 shoots and fresh leaves. The shoots had much the appearance of hav- 

 ing been "forced," and were 8 cm. more or less, in length. Thus it 

 was shown that relatively great vegetative activity of Opuntia versicolor 

 can be induced under unfavorably low conditions of air temperatures if 

 the roots are kept at temperatures favorable for their growth. This 

 result constitutes a rather interesting confirmation of the conclusions of 

 Halsted and Waksman, although having reference to quite a different 

 tj'pe of plant, and further emphasizes the importance of including soil 

 temperatures in any definition of the physical characteristics of the 

 habitat of a species. — W, A. Cannon. 



