248 The Plant World. 



presence or absence, life history, form, and structure of the p'lant 

 in a given habitat, these components may balance or ofiset each 

 other to a certain extent. For example, the hi^h temperature 

 of a canyon at low altitudes appears often to beoiTset in the plant 

 economy by an abundance of moisture. W hat all the difierent 

 components are, and especially how they act, is for the [thysiol- 

 ogist to determine. 



The species of widest local distribution, v, hich are frequently 

 also those possessing, for their size, the largest number of indi- 

 viduals per square mile, are usually those exhibiting the largest 

 number and the widest range of variations. It is this versatility 

 or comparative plasticity that must enable such a plant to 

 achieve predominance over its fellows, and to extend or hold 

 its range. But even to it, no considerable or rapid extension 

 of its range appears possible, nor will it on the other hand be 

 likely to lose its ground while the present geologic age and 

 climatic conditions endure. 



Tucson, Arizona. 



BOOKS AND CURRENT LITERATURE. 



Plant-Animals. — The little book under consideration* 

 brings to the layman in popular form the contents of sev- 

 eral scientific papers on two worms belonging to the genu^ 

 Convoluta. It consists of two parts, one devoted to th:* 

 structure, activities and general habits of these animals, 

 the other to the relation between them and the symbiotic 

 algae which they contain. 



Among the most interesting characteristics in the 

 behavior of the Convoluta is the periodic activity they have 

 acquired in correspondence with that of the tides. This is 

 especially marked in the green species C. roscoffensis which 

 is found only on certain sandy beaches of Normandy and 

 Brittany just below the high- water mark of the slackest 

 neap tide. When the tide is out they are found on the sur- 

 face forming a thin green film, but as soon as it comes in 

 they crawl into the sand and disappear from vieAV. Twice 



•Plant- Animals :— A Study in Symbiosis, by F. Keeble, 163 pp. Cambridge, 

 1910. 



