50 ROOT HABITS OF DESERT PLANTS. 



a boulder was perceptibly wet to a depth of 25 cm., while less than i meter 

 distant the soil, free of large rocks, was air-dry at this depth. 



The root-system of the older plant, which grew on the western side of 

 Tumamoc was different in some particulars from that of the plant just 

 described. The shoot of the plant consisted of two main branches and 

 three secondary ones, with a stout central axis, and the weight of all of the 

 branches was born by the stem. There was no main or tap root, as in 

 other cacti, but several roots, constituting a brush, left the base of the stem 

 and penetrated the ground at an acute angle. Of these roots, the longest 

 attained a depth of 15 cm. in a horizontal direction. From the central 

 group there arose 8 leading roots which formed the superficial portion of 

 the system; these varied in length from 50 cm. to 1.3 m.; the longest were 

 those extending uphill. The superficial roots varied greatly in depth, some 

 being as close to the surface as 2 cm. and others penetrating as deep as 20 

 cm., but the deeply placed ones uniformly ran underneath boulders, while 

 the more shallow roots were situated where the soil was free from large 

 rocks. The bases of the superficial roots were somewhat enlarged; for 

 example, a root 50 cm. long was 1.5 cm. in diameter at its base, but the 

 roots as a whole were ver}^ slender. The enlargement of the bases of the 

 roots probably represents a regulatory response on the part of the roots 

 by which the weight of the shoot is borne and the strains incident to winds 

 successfully withstood. Thus roots functioning primarily as absorbing 

 organs become, in the absence of a well-developed tap root, the most 

 important mechanical support of the heavy shoot. 



The superficial roots of the cactus favor absorption of water even when 

 the rains have been slight, as maybe illustrated by one or two instances. On 

 October 1 7 half an inch of rain was recorded as having fallen at the Desert 

 Laboratory. Within 3 days the joints of Opuntia discata lost the shriveled 

 appearance which they had during the dry season and became plump. On 

 another occasion the response was noted within 24 hours after a rain of 0.54 

 inch; also, 24 hours after irrigating a certain plant it showed by its increased 

 turgidity that it had taken up water. In none of these instances had the 

 water penetrated sufficiently to be of benefit to plants the absorbing system 

 of whose roots was 10 cm. below the surface. 



Opuntia Leptocaulis. 



Opuntia lepiocauHs grows mainly and was studied only on Tumamoc Hill ; 

 it has very slender shoots, as is indicated by the specific name, and has 

 certain biological relations, probably associated with the character of the 

 shoot, which are of much interest. 



As a rule the cacti in the vicinity of the Desert Laborator}' do not hold 

 close and fixed relations with other plants ; they grow near or remote from 

 other plants, apparently wholly according to chance. But the species 

 leptocaulis is a ver}^ striking exception to this, in that it rarely occurs alone 

 or remote from other and larger forms. The shoot habits of Opuntia vary 



