November 20, 1914] 



SCIENCE 



751 



and setting down in a concise and clear form 

 the results of many years of very successful 

 study and teaching of the subject presented. 

 It may well become a new starting point for 

 editions that should take on the size and type 

 of illustration that the dignity of a college 

 botany deserves. Here is a hearty welcome 

 to the new text-book in phytophylogeny — The 

 Besseys' Botany Book of Branches. 



Byron D. Halsted 

 EuTGERs College, 

 October 28, 1914 



Botanical Features of the Algerian Sahara. 

 By William Austin Cannon. (Publication 

 No. 1Y8, Carnegie Institution of Washing- 

 ton. 1913. 81 pages, 36 plates.) 

 The journey of which this paper is an ac- 

 count was made in order " to examine the 

 more obvious features of the physiological 

 conditions prevalent in the region in question 

 and, in connection with these observations, to 

 make some detailed studies of the root-habits 

 of the most striking species of the native 

 flora." 



After introductory chapters on the geog- 

 raphy and climate of Algeria, the writer pro- 

 ceeds with an itinerary of his trip through the 

 desert. This portion of the paper contains a 

 great deal of topographical detail, together 

 with much that is of directly botanical inter- 

 est, although presented in a somewhat desul- 

 tory way. The important botanical data are 

 treated more systematically in the " General 

 Summary and Conclusions" (pp. 66-81). 



The author's intimate acquaintance with 

 the vegetation of the southern Arizona deserts 

 makes his comparison of conditions there and 

 in the Algerian Sahara of special interest and 

 value. Some of the striking points of differ- 

 ence as summarized in the concluding para- 

 graphs are: (1) the greater sparseness of the 

 Saharan vegetation, as compared with that of 

 Arizona, there being " probably no large area 

 in southern Arizona, where the soil conditions 

 are favorable for plants, where the water con- 

 ditions are too meager to support a perennial 

 flora of some sort. The greater aridity of the 

 northern portion of the Sahara is evident. 



therefore, from the great contrast in its flora." 

 Cannon therefore suggests the term " semi- 

 desert " for the Arizona region in contrast 

 with a true " desert " like the Algerian Sahara. 



(2) The smaller size of the individual plants, 

 at least of the perennial species, in the Sahara. 



(3) The smaller development of spines. " What 

 may be the proportion of armed to unarmed 

 plants in the northern Sahara I do not know, 

 but to a person familiar with the plants of 

 southern Arizona, where spinose forms are very 

 numerous, the Algerian plants do not appear 

 especially well protected." 



Attention is also called to the fact that 

 while in the Arizona desert there are numer- 

 ous species, among the Cactacese and other 

 families which have a "water balance," i. e., 

 which during and immediately after rains 

 store water in their tissues, to be dravm upon 

 in periods of drought; few examples of this 

 adaptation were met with in the Algerian 

 Sahara. Cannon correlates this scarcity of 

 "water balance" plants with the fact that in 

 Algeria there is but one rainy season. He notes 

 that in the Tucson region, where such plants 

 are numerous, there are two rainy seasons dur- 

 ing the year, while in the desert region farther 

 west, where but one well marked rainy season 

 occurs, succulent plants are few or wanting. 



The author's studies of the root habits of 

 desert plants in Arizona led him to devote 

 especial attention to this feature of the 

 Saharan vegetation. The results of his in- 

 vestigations are summed up as follows : " A 

 study of the relation of the root-type of the 

 Algerian plants to the plant's distribution 

 leads to the same general conclusion already 

 obtained by similar but more extended study 

 in the Arizona desert, namely, that the con- 

 nection is often a very close one and often of 

 definitive importance. Where the root-type is 

 an obligate type the distribution of the spe- 

 cies is much restricted, but where it undergoes 

 modification with changed environment the 

 distribution of the species is much less con- 

 fined. It is of interest to note especially that 

 as a rule it is the latter kind of root system 

 that is developed by such plants as occur 

 where the soil conditions are most arid, that 



