238 PLANT PHYSIOLOGY 



tage of providing a constantly increasing curvature of surface, per— itting 

 the critical angle, where the influence of geotropism overpowers that of hydro- 

 tropism, to be determined; and, moreover, by use of different sizes of pots, 

 it allows surfaces of any desired curvature to be obtained. If a pot be filled 

 with sphagnum kept saturated, and if seeds te planted around its edge with 

 their young roots projecting over, these will follow the sloping surface of the 

 pot very beautifully. Instead of the pots one may use the low cylinders 

 formed by Zurich germinators, two of which placed edge to edge and filled 

 with wet sphagnum form an excellent arrangement, as described in the first 

 edition of this book (page 130). The usual way of demonstrating hydrot- 

 ropism is that introduced by Sachs, consisting of an inclined trough of 

 wire netting filled with sawdust or soil containing the seeds, the roots of 

 which issue from the netting, but keep close to the moist slope. Another, 

 but inferior, way commonly recommended lies in the use of a funnel con- 

 taining soil or sawdust, and covered with filter-paper kept wet by water in 

 a supporting bottle, the seedlings being started with their roots over the edge. 

 In the use of all of these methods, the regulation of the humidity of the sur- 

 rounding air is very important; for if this is kept at saturation, the hydrot- 

 ropism is neutralized and only geotropism prevails, while if too dry the 

 roots die at the tips. The humidity can best be regulated by use of a bell 

 jar open at the top; this can be raised upon blocks, or other support, a 

 little from the table until the right amount of humidity is found. A method 

 of showing hydrotropism of roots in the soil is given by Osterhotjt, 96. 



The results of this experiment upon roots will at once suggest 

 the correlative inquiry as to the hydrotropism of stems, upon 

 which matter the student may now be left to devise and carry 

 out a good experimental test for himself, if, indeed, the preceding 

 experiment does not settle this point. And he should now com- 

 plete his knowledge of hydrotropism by aid of the literature. 



(d) Thigmotropism. 



Responses to contact, or to mechanical shocks of various 

 kinds, are fairly familiar in plants. The student should now 

 plan (with some suggestions from Detmer and from Darwin 

 and Acton) and carry out a series of experiments and observa- 

 tions directed to show the facts as to: 



(a) The responses of tendrils (and some twining and parasitic 

 stems) to contact (Haptotropism). 



(b) The responses of the Sensitive plants (Mimosa and others) 

 to various kinds of stimuli, also of Venus' Flytrap and Sundew. 



(c) The responses of stamens of Barberry, Opuntia, etc., and the 

 stigmas of Mimulus. 



