ROOTS AND RHIZOIDS 



509 



Much has been said concerning the advantage of cypress knees and of horizontal 

 swamp roots, the prevalent view being that respiration is facilitated by these 

 structures. The efficiency of cypress knees as aerating organs is not known ; 

 although their tissues are rather loose, the knees are covered with bark, and often 

 are clothed with dense layers of mosses and liverworts, which must retard the pene- 

 tration of gases. Furthermore, knees are absent in deep water, where aeration is 

 most needed. The shallow horizontal root system of the tamarack seems disad- 

 vantageous in part, since it does not afford adequate anchorage; severe storms 

 may overturn these trees in considerable numbers. Probably horizontal surface 

 roots in swamps are of real advantage in facilitating aeration, though this view is 

 based upon assumption rather than upon experiment. 



The plank roots of tropical trees, resembling boards on edge, are due to excessive 

 growth on the upper sides of soil roots; the causative factors are unknown, but it 

 is possible that growth is freer in the air than in the soil. An analogous phenome- 

 non, due chiefly to stem activity, is seen in various swamp trees, as the tupelo and 

 the bald cypress, and to some extent in the elm, the base of the trunk being greatly 

 enlarged (fig. 726). Possibly enlarged buttressed trunks and plank roots are of 

 advantage in holding trees in place where the soil roots are relatively inefficient as 

 anchorage organs. 



Correlation of roots and leaves. The ecological behavior of a plant cannot be 

 determined from one set of organs. For example, the hemlock and the red cedar 

 have somewhat similar leaves, being small, thick-skinned, and apparently fitted to 

 withstand excessive transpiration. But while the red cedar can thrive in very xero- 

 phytic situations, the hemlock thrives best in mesophytic woods with such large- 

 leaved trees as the sugar maple. It is believed that the differing habitats of these 

 two trees is a matter of correlation between roots and leaves, the red cedar having 

 an extensive root system and the hemlock one that is more meagerly developed. 

 Jt^ would appear that the ratio between absorption and transpiration determines the 

 habitat for which a plant is fit. It should be emphasized, however, that the lack of 

 experimental evidence makes this theory of correlation as yet merely a plausible 

 hypothesis (see also p. 747). 



2. WATER AND AIR ROOTS; 

 RHIZOIDS 



Water roots. The most repre- 

 sentative water roots are found in 

 plants that are not attached to a 

 substratum, such as the duckweeds 

 (fig. 727) and the water hyacinth. 

 Such roots are not numerous nor 

 large, and in most cases branches are 

 few and root hairs wanting ; in some 

 duckweeds there is but a single small 



FIG. 727. Plants of a duckweed 

 (Spirodela polyrhiza) floating free on 

 the water, showing flat thalloid shoots, 

 from each of which there depend 

 several water roots. Each thallus or 

 frond represents a single individual hat 

 has arisen from another such frond 

 vegetatively ; the mother and the daugh- 

 ter fronds remain attached for a time 

 in colonies. 



