34 



THE AMERICAN NATURALIST 



[Vol. XL 



are slender throughout their entire course and they are super- 

 ficially placed. 



The roots of Oereus giganteus, on the other hand, in form and 

 position, and perhaps in extent and branching also, are very dif- 

 ferent from those of Echinocactus. Fig. 5 represents the root 

 system of a Cereus giganteus, about one meter high, which was 

 growing 200 meters west of the Echinocactus just described. Four 

 main roots were observed to arise from the base of the plant. At 

 first they were relatively heavy, from 2 to 4 cm. in diameter at 

 the proximal ends, but they became smaller very rapidly as the 

 distance from the plant increased — in a manner much as is 

 indicated by A of the figure. Very soon after leaving the plant 

 the roots branched. One branch, whose later history could not be 

 traced, struck directly downwards, and the other took a more or 

 less horizontal course. The latter branched at intervals, although 

 perhaps not so frequently as those of Echinocactus, and extended, 

 in one instance at least, over one meter from the plant's base. 

 How much farther the root reached could not be learned because 

 of its fragility and the small size of the distal branches. The 

 superficial portion of the root system of rV/w/.v (jigavirus was 

 more deeply placed than were the roots of Kcliinocnctiis, ami 

 owing to the fact that these parts were not so richly hraiiclicd, 

 the ground included by them was not so thoroiiij;lil v ( (ivcn-d. 

 However, in' one characteristic, which is of interest to note but 

 whose significance I have not investigated, the snperfieial roots of 

 the two forms are alike, namely, the longer roots and the L,n-eatest 

 number of roots are situated on the uphill side of the respective 

 plants. This peculiarity is shown in the two figures. In Fig. 4 

 the uphill side is to the right, and in Fig. 5 it]is^at the toj) of the 

 sketch.. 



Certain Structurat. Features 



Perhaps the correlation of -structure and function is nowliere 

 more patent than in tlie j)ecuhar Iran-piration-coiitroUinu- and 

 transpiration-promoting tissues, and the rate of transpiration in 

 certain cacti. As is well known, the cacti are well adapted struc- 

 turally, laying aside for the moment the matter of water storage, 



