REVIEWS 991 



of Colorado, (5) the sandhills anti-climax, (6) the gravel-slide, (7) the 

 iialf-gravel-slide, (8) forest community of the Rocky Mountains of 

 Colorado. 



Trenches were dug to a depth of about 6 feet by the side of the 

 plants to be examined. These trenches were deepened where necessary 

 as the investigation proceeded, sometimes to a depth of 16 or more 

 feet. The author states that by use of a hand-pick with a cutting edge, 

 practice made it possible to excavate a root system almost in its entirety. 

 The reviewer has found in his work on root systems that with young 

 trees two or three years old it is almost impossible to remove the root 

 systems from heavy clay and stony soils without breaking off many of 

 the tender root tips. 



In all cases the roots examined by Weaver were those of mature 

 perennial plants. A large number of excellent line drawings and pho- 

 tographic reproductions supplement the descriptions. 



The description of representative root systems from each of the 

 eight communities is followed by a more or less complete account of the 

 climatic and edaphic factors of the site where the root systems 

 developed. 



In the prairie environment of eastern Nebraska 55 per cent of the 

 species investigated had root systems extending to a depth of 5 feet or 

 more. The cause of this remarkable development of shrubs and herba- 

 ceous perennials is assigned to the disposition in the soil of available 

 \vater in the prairie environment. The writer states that the prairie 

 plants of this region grow under semi-arid climatic conditions in which 

 the supply of water is the chief limiting factor of plant growth. Dur- 

 mg certain parts of the growing season it was found that the water 

 content of the soil may be reduced to the non-available point to a depth 

 of 4 or 5 feet, and at times when the evaporating power of the air is 

 very high. The very deep and extensive root systems are a response to 

 deep soil moisture at times of drought. 



Without attempting to review the author's studies and conclusions 

 from six of the communities examined, all of which are of great in- 

 terest, the review of his study of the forest community in Colorado is 

 presented as a contrast to the prairie community. The forest com- 

 munity studied is represented by Pimts ponderosa and Pseudotsuga 

 miicronata as dominants, both of which are frequently preceded by a 

 chaparral stage. The root systems of a large number of herbs and 

 shrubs characteristic of the more mesophytic type of forest were 

 examined. 



