Minnesota Plant Life. 



439 



of more northern regions similarly perch themselves upon the 

 trunks, or branches, of trees ; and with these, too, a number 

 of mosses and liverworts will be found. Sometimes the perch- 

 ing-plant adroitly selects a position where it will receive more 

 moisture than elsewhere. So, many mosses grow around the 

 bases of tree trunks, for the tree with its branches serves as a 

 drain, by which the water of rains is brought to the position 

 pre-empted by the moss. Lichens and mosses alike select the 

 degree of illumination that they prefer, and in northern lati- 

 tudes arrange themselves on the southern sides of tree trunks 

 if they require stronger illumination, but on the northern sides 

 if their requirements lie in the other direction. Some perching 

 plants acquire the habit of driving their roots into the branch 

 upon which they stand. Thus originated the parasitism of the 

 mistletoe and related plants, quite a different method of its 

 development from that shown by the cancerroots, which learned 

 to clutch the roots of neighboring plants and drive little suck- 

 ing organs into their soft and nutritious tissues. 



Another result of the mutual proximity of plants is the 

 development of climbing or twining species. Some climb by 

 means of prickles, thorns or hooks, and thus the brambles or 

 tear-thumbs lift themselves upon surrounding vegetation. 

 Others, like the scouring-rushes, brace themselves by means 

 of lateral branches and lift their slender stems farther into the 

 air than they could without assistance. The clematis vines 

 twist their leaf stems around twigs that chance in their way and 

 thus show a tendency toward tendril production. Other vines, 

 like the smilaxes, the grapes, or the wild cucumbers, develop per- 

 fected tendrils, and by aid of these lift themselves high up on the 

 stems or branches of neighboring trees. The bittersweets, 

 morning-glories and hops learn to roll spirally their slender 

 stems around the shrubs that stand near them, thus twining 

 to a considerable height. 



All such habits must have arisen by degrees, and each of 

 them, when accentuated, might encourage dependent habits 

 of nutrition, finally resulting in parasitism. Thus, the twining 

 habit of some morning-glory vine may have given, in some 

 earlier epoch, opportunity for the production of the parasitic 

 dodders. 



