ROOT HABITS OF DESERT PLANTS. 35 



of available moisture near the surface of the ground. The leading feature 

 of the root-system is the long and slender tap root which carries numerous 

 rather short filamentous laterals. The general character of the root-system 

 is shown in plate 6. The length of the tap root varies with the age of the 

 plant from 5 cm. in one 4 cm. high to over 9.5 cm. in a plant with a shoot 8.5 

 cm. long. The laterals of the first order are usually less than 2 cm. long, 

 and branch infrequently, so that as a whole the root-system corresponds 

 very well with the "extensive" type of Biisgen* (plate 6). 



Malv.\ Borealis. 



Maha, one of the most common of the winter annuals, was studied on 

 Tumamoc Hill only, although it occurs on the flood-plain of the Santa Cruz 

 also, where it is perhaps most abundant along the irrigating ditches. The 

 root-S}^stem of the mature plant is characterized by the prominence of the 

 laterals, which are coarse and relatively long. Several characteristic changes 

 should be noted, leading to the formation of the mature root-system as 

 shown in plate 6. 



In the development of the root-system of Malva there are two fairly well 

 differentiated stages, of which the immature is very different from the 

 mature. The seedling plant sends down a tap root which penetrates the 

 soil to a depth of about 8 cm. before laterals become prominent, or indeed 

 appear in numbers. The prominent tap root, therefore, is the leading 

 feature of the seedling. As the plant increases in size the tap root strikes 

 down quickly and strongly until a condition shown in plate 6 is reached. 

 After this the laterals begin to grow and from this time they constitute the 

 chief feature of the system. At first the laterals arise singly, and close to 

 the surface of the ground, but in time they become of importance by their 

 increase in length, by the formation of branch roots of the second order, and 

 by the formation secondarily of laterals of the first order along the main root 

 axis. But the tap root is always an important feature, since in the mature 

 plant it may attain a length exceeding 16 cm. 



The secondary formation of laterals of the first order, by which groups of 

 three or more roots are formed, which are of unequal age, has also been 

 noticed in several other annuals and probably also occurs in many peren- 

 nials. This not uncommon character of plants must greatly increase the 

 efficiency of the root-system by adding much to the absorbing surface 

 without greatly increasing the distance of water transport or of bringing the 

 roots into more active competition with those of neighboring forms. 



Medicago Denticulata. 



Mcdicago, one of the introduced forms which have their period of greatest 

 activity in winter, is abundant on the flood-plain of the Santa Cruz. The 

 plant examined, however, was growing on Tumamoc Hill in the vicinity of 

 the Desert Ivaboratory. Mcdicago has a generalized type of root-system, 



♦Compare von Alten, Wurzelstudien, Bot. Zeit., vol. O7, p. 192, igog. 



