PHYSIOLOGICAL FEATURES OF ROOT-SYSTEMS. 

 •CHARACTER' IN ROOTS. 



The roots of desert plants, possibly to a greater extent than the roots 

 of plants of the more humid regions, are remarkable for their individuality. 

 The roots of each genus, often perhaps of each species, possess peculiarities 

 of form, of branching habits, of color, of texture, of position in the ground, 

 or of more subtle physiological reactions. In how far these features can 

 be used for systematic purposes is yet to be shown, but there is no question 

 of their importance in the field of plant biology, particularly in ecology. 



It has been found convenient to group the types of root-systems into 

 generalized and specialized forms with a physiological rather than a sys- 

 tematic bearing. Bv a generalized root-system is meant one that has both 

 the tap root and the laterals well developed. ,Such roots penetrate the 

 ground deeply and reach out widely. To this type belong most of the 

 desert perennials, of which I'^iaKsciia deltoidea and Prosopis veluiina, among 

 the shrubs and trees, may be taken as representative. Nearly all the 

 annuals, also, belong to this type. There are two forms of specialized 

 roots, those with the tap root the chief feature, as in Ephedra tnfurca and 

 Kacrhcrlinia spinosa. and those with the laterals, which are placed near the 

 surface of the ground, especially well developed. Oi the latter, the cacti 

 are almost the sole representatives. So far as desert plants are con- 

 cerned, it is probably true that generalization in root-systems looks toward 

 mesophilv, and specialization toward xerophily. 



The specialized root-systems of either form are changed little with envi- 

 ronment, but the generalized roots a e often extremely variable, ranging 

 from a pronounced ta]i root to a marked development of the laterals, depend- 

 ent on soil characters and water relations. Thus it will appear clear that 

 rigidity of plasticity of root-system may be an important factor in the 

 local distribution of a species, a feature to be discussed below. 



One of the most striking characters of the root-systems of perennials is 

 the variation in the branching habit, although as the habit is a matter of 

 degree rather than of kind it is not possible to express the difference in 

 exact terms. Under parallel conditions, the roots of any species act con- 

 sistentlv, and where the conditions are much changed, as from the bajada 

 to the flood-plain, and modification of the branching habit results, still the 

 induced variation is superimposed on the specific and familiar habit in such 

 a way that the proper habit is easily recognized. The roots of Covillca 

 branch repeatedly, wherever the plant is growing, but those of Foiiquieria 

 are little branched. The most richly branched root-system observed was 

 of Opuiilia arhusculci, which covered the ground so completely that it would 



