Minnesota Plant Life. 179 



growth-conditions it is capable of branching ahnost as abun- 

 dantly as the field horse-tail. A curious dwarf variety two or 

 three inches high is sometimes found growing in tufts in deep 

 woods. It is reported from the St. Croix river valley, but I 

 have not seen authentic specimens of it from Minnesota. 



Underbrush habits of horse-tails. When the branched va- 

 rieties of horse-tails grow in the edges of woods they often be- 

 come very much taller than in fields. This they accomplish by 

 thrusting out their rigid side-branches in every direction and 

 permitting them to rest upon the twigs of surrounding shrubs 

 or herbs. Thus they can distribute their weight in such a way 

 that the main stem is relieved and the axis may extend itself 

 vertically farther than otherwise. Plants which lean in such 

 fashion upon surrounding plants are known as braced-plants. 

 They are not exactly dependent for their well-being upon the 

 presence of other plants as are the climbers and twiners, but 

 they do derive some advantage from their habit of letting a 

 portion of their weight rest upon plants near them. 



It is really, if one stops to think of it, quite as much of an 

 engineering problem to erect a slender stem as to build an Eifel 

 tower, and it is no less impossible to extend a leaf into the air 

 without due regard to the strength of materials than it would 

 be to build a cantilever bridge from wet paper. Plants mani- 

 fest architectural design and the problems of structural engi- 

 neering are not at all unlike those requiring solution by the 

 human architect or bridge-builder when he enters upon the 

 plans of a new structure. So it is obvious that the bracing of the 

 side branches of horse-tails, thus diminishing the strain upon 

 the main axis, might enable it under the same general type of 

 structure, to reach a greater elevation into the air. In South 

 America, by bracing devices scouring-rushes grow to a height 

 of twenty or thirty feet, though they are not thicker than an 

 ordinary walking-stick. Where the forest is dense and dark 

 such a plan is seen to be highly advantageous and perhaps even 

 necessary, but in the lighter, thinner forests of Minnesota there 

 is no need of such extreme length. 



