ECOLOGIC SUBTERRANEAN ANATOMY OF PLANTS 9 1 
Before applying these tests perhaps the reasons for selecting them should 
be given. An examination of young water-absorbing roots shows them to 
have a deep cortex hearing root hairs and a small vascular cylinder with dis- 
tinctly radial bundles. "In roots, any departures from the typical radial 
structure of the vascular strands are generally correlated with special en- 
vironmental conditions, or arise from the necessity of increasing the amount 
of available conducting tissue" (Haberlandt). In a radial root there is no 
means of tangential increase, so this increase must take place in a radial 
direction toward the cortex and results in orienting a cambium which pro- 
duces concentric layers of phloem and xylem. This is seen to be an ad- 
vantageous structure in older roots whose function is conduction and not 
absorption from the cortical layer, for here there is no incoming stream of 
water to cross the proteid-conducting zone but only a rising central column. 
It is seen that such an arrangement is also desirable for resistance to strains 
which in roots are in a longitudinal direction. This solid cylinder gradually 
develops pith and assumes an annular vascular structure in the stem, from 
which bundles shoot out into the branches. Hence a raot changes from a 
water-collecting to a water-radiating organ and the pith of the stem serves 
as a good collecting reservoir; though pith is sometimes absent. The stem, 
being subject to radial strains, is thus well adapted by its hollow-cylinder 
mechanical system. 
Rhizomes (Haberlandt) which fix the plant in the soil agree with roots 
in having their mechanical tissues united to form a stout axile tube or a 
solid central strand; this centralization of the mechanical system is very 
marked in the rhizomes of grasses, sedges, and rushes, which, accordingly, 
when regarded from an anatomic-physiological standpoint, approximate 
more closely to roots than to the aerial stems of which they are the morpho- 
logical equivalents. 
Structural features (Solereder) which vary with the amount of water in the 
soil and air and with the degree of transpiration on the part of the plant, 
affecting chiefly the number of vessels and width of lumina, are of minor 
systematic value. 
Kohl observed that certain plants {Mentha aguatica, Thalictrum gale- 
oides, and Menyanthes trifoliata) develop more collenchyma and bast if 
grown in dry air, i. e., under conditions favorable to transpiration, than 
they would if produced in a moist atmosphere, i. e., with their transpiration 
reduced. Here it is impossible to state with certainty whether the process 
is adaptive or self-regulatory. It should, however, be noted that in the 
case of herbaceous plants growing in a dry atmosphere, or in fact under 
xerophytic conditions in general, turgor has a smaller mechanical value 
than usual because the risk of temporary wetting is so great in these cir- 
cumstances that any decrease in the development of mechanical tissue 
must be advantageous. In general, there is correlation between the number 
of water-conducting vessels and the extent of the foliar transpiring surface. 
