EFFECT OF NON-MECHANICAL AC4ENCIES 197 



itself to artificial mechanical influences, unless the latter accentuate, or 

 at any rate imitate, the natural mechanical forces to the action of 

 which the organ under investigation is normally exposed. In other 

 words, only such structures as roots, tendrils and pendulous i'rnit-stalks, 

 should be subjected to continuous longitudinal tension. Stems and 

 petioles should be bent to and fro ; for under natural conditions 

 inflexibly constructed organs are never continuously exposed to longi- 

 tudinal tension. 



The development of the mechanical system may be affected by 

 influences which are themselves not of a mechanical nature at all. 108 

 We are indebted to F. Haberlandt for some observations concerning 

 the influence of the humidity of the soil, which furnish incontrovertible 

 proof of the favourable effect of watering upon the strength of the 

 bast-fibres of Cannabis sativa. The average load required to break a 

 strip of bast 2 mm. in width was 4" 12 kg., if the fibre was taken from 

 unwatered plants ; in the case of plants growing in well- watered soil, 

 the corresponding load was as much as 5 '4 8 kg. In this experiment, 

 however, it was left undecided, whether the increase of strength caused 

 by watering was due to an increase in cross-sectional area of the 

 fibrous strands, or to increased resistance on the part of the cell-walls. 

 Kohl, on the other hand, has observed that certain plants (Mentha 

 aquatica, Thalictrvm galioides, Menyanthes trifoliata, etc.), develop more 

 collenchyma and bast if grown in dry air i.e. under conditions favour- 

 able to transpiration, than they would produce in a moist atmosphere 

 i.e. with their transpiration reduced. Here, again, it is impossible to 

 state with certainty whether the process is adaptive and self-regulatory 

 or not. It should, however, be noted that in the case of herbaceous 

 plants growing in a dry atmosphere, or in fact under xerophytic con- 

 ditions in general, turgor has a smaller mechanical value than usual, 

 because the risk of temporary wilting is so great ; in these circum- 

 stances, any increase in the development of mechanical tissue must be 

 advantageous. 



VI. THE MECHANICAL SYSTEM AMONG THE 

 THALLOPHYTA. 



Fresh- water Algae that grow in running water, and Sea-weeds that 

 are exposed to the action of the waves, must evidently be possessed 

 of inextensibility and shear-resistance ; otherwise they could not long 

 withstand the stresses to which they are constantly subjected. 

 According to Wille 109 the FuCACEAE are actually provided with special 

 thick-walled mechanical cells, which are highly extensible and elastic : 

 these occur mainly in the stalks and midribs, where they are disposed 



