ROOT HABITS OF DESERT PLANTS. 

 In presenting descriptions of the root-systems of the desert plants, it 

 seems best to be guided by the biological grouping — that is, to bring plants 

 together which are naturally segregated either in time or in space. Accord- 

 ingly perennials which arc typical of the leading habitats treated are 

 grouped under the respective habitats, but since the annuals of winter and 

 of summer occupy the same areas and are separated only by time they are 

 divided into winter and summer forms. By summer annuals is meant 

 those which appear in midsummer, and by winter annuals those which 

 come after the w^inter rains, although some of them might more truly be 

 called spring annuals than those of winter. 



SUMMER ANNUALS. 



In early July the different habitats show only perennials, and the bare 

 ground between them shows the dried remains of the annual vegetation of 

 the preceding rainy season. With the coming of the rains of late July or 

 August, however, the appearance of the land is suddenly changed. Seeds 

 which have lain dormant for nearly ii months promptly germinate, and 

 hill and plain are speedily clothed with a growth of evanescent forms which 

 are in great variety and, for desert forms, of surprising density. 



The study of the summer annuals was carried on in the following manner. 

 The roots of typical forms, mostly mature, were carefully washed out by a 

 small jet of water, and the entire plant, or as much as was recovered by this 

 means, was preserved in weak formaline for subsequent study. By this 

 method the entire root-systems were rarely to be had, but enough was 

 always saved to give the general character. More than one plant as a rule 

 was preserved of each species so that a control on the observations might be 

 had. A very serious drawback to this method of study lay in the impos- 

 sibility of surely determining the extreme depth to which the main root 

 (or the laterals) penetrated the soil, although in many cases this feature 

 was learned with close accuracy. The following summer annuals were 

 examined : 



Amaranthus palmeri S. Wats. Dysoda papposa Lag. 



Anoda thurberi Gray. Euphorbia glyptosperma Engelm. 



Aster tanacetifolius HBK. Kallstroemia grandiflora Torr. 



Boerhaavia sp. Pectis prostrata Car. 



Cladothrix lanuginosa Nutt. Solanum ela;agnifolium Cav. 



Datura sp. Trianthema portulacastrum L. 

 Ditaxis humilis (Engelm. and Gray) Pax. Vicia sp. 

 (perennial). 



Amaranthus Palmeri. 



The species of Amaranthus studied was growing near the Laboratory 

 building and was removed from the soil on August lo, at which time the 

 shoot was 21 cm. long. The plant was not in flower. 



The root-system consists of a tap root and several large laterals which 

 bear filamentous branches. The roots do not intergrade in size, certainly 



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