420 



SCIENCE 



[N. S. Vol. XXXVII. No. 950 



forms the backbone of Java, the intervening 

 Sunda Strait being only fifteen miles across, 

 the existing flora and fauna of Java are less 

 like those of Sumatra than those of the latter 

 are like those of Borneo. The biota of Java 

 is, on the other hand, much more like that of 

 the Siamese peninsula and northern India, 

 and it is very interesting to find similar and 

 apparently anomalous aifinities shown as long 

 ago as the Pleistocene and certainly before the 

 submergence which gave the region its pres- 

 ent physical geography. 



Edward W. Berry 

 Johns Hopkins University 



SPECIAL ARTICLES 



SOME RELATIONS BETWEEN ROOT CHARACTERS, 

 GROUND WATER AND SPECIES DISTRIBUTION 



Observations on the root habits of desert 

 shrubs indicate that the root-type of any spe- 

 cies may be of importance in limiting the dis- 

 tribution of the species. This has been found 

 to be especially clear in the case of plants 

 having obligate tap roots, which, as a rule, are 

 confined to relatively deep soils. Such shrubs 

 as have a generalized root-system, on the other 

 hand, have a wide local distribution, which 

 may be correlated with the fact that the roots 

 of these plants are capable of a large degree 

 of modification in conformity with the pres- 

 sure of the soil environment. But the role of 

 the superficial type of roots, such as is typical 

 of species with water storage capacity, is not 

 so well defined. It is known that the fleshy 

 cacti, for example, are most highly developed 

 where the rainfall is a periodic one, occurring, 

 perhaps, twice each year, but that these plants 

 occur sparsely where the precipitation takes 

 place once annually. Whatever may be the 

 reason for this limitation, it is noteworthy 

 that the larger mass of absorbing roots of 

 species having a water balance lie within 10 

 cm. of the surface of the ground. The super- 

 ficial soil layer is subject to the most intense 

 desiccation, and, hence, carries moisture in 

 sufficient amount for the use of plants for the 

 shortest period only, so that plants depending 

 on this stratum for moisture must either be 



short-lived or have the capacity of storing up 

 water against the following period of drought. 

 What the minimum absorption time of fleshy 

 plants is, has probably not been determined, 

 but it is evident, from their distribution, that 

 the amount of available moisture in the super- 

 ficial soils derived from a single rainy season 

 each year is not sufiicient. To put the case 

 in another way, it is apparent that the general 

 and local distribution of the fleshy cacti would 

 be other than it now is, if such plants had 

 another type of root-system, for instance, if 

 there was an obligate deeply penetrating root- 

 system, in place of the superficial one they 

 now have. Such a change, were it possible, 

 would, in the first place, limit the local dis- 

 tribution to flood plains or to other areas 

 having deep soil, and, in the second place, it 

 would permit a wider general distribution. 

 This suggestion makes it evident that the 

 root-soil moisture relation may be an impor- 

 tant factor among those which determine the 

 survival of a species. 



Such observations as have been made on the 

 root habits of trees indicate that in these 

 large-bodied plants the root character may 

 also be of importance among the factors which 

 operate to influence their distribution. 



It is now well established, at least for a por- 

 tion of the Southwest, that there may be a very 

 intimate relation between the occurrence of 

 certain species of trees and the character of 

 their roots, having regard to the depth at 

 which perennial water may be found. Here 

 trees occur along streamways, while the near- 

 by upland may be treeless. The humidity of 

 the two areas may not be very unlike, nor the 

 rainfall, nor yet the temperature. The great 

 difference, which is often striking, lies mainly 

 in the soil conditions, particularly with regard 

 to the depth to the ground water. On the 

 bottoms the water table lies within reach of 

 the roots of trees, while on the more elevated 

 land it is far below them. 



The depth to the level of ground water, or 

 to the soil that is moistened from the water 

 table, is usually not great. In the eastern 

 portion of the United States, in lands of mod- 



