292 



BOTANY 



PART I 



buds gravity must also play a part. It has, however, in no case proved possible 

 to effect a complete and lasting inversion of the polarity of a plant in this way ; 

 while such inverted plants may live for a considerable time, they exhibit serious 

 disturbances in their anatomical construction (*"). 



An effect of gravity on the internal disposition is also seen in the case of 

 obliquely or horizontally placed branches. The tendency of the internal disposition 

 is to cause the uppermost buds to develop and give rise to long 

 shoots. On branches displaced from the vertical the basal buds are 

 favoured and the more apical buds arrested. When the branch 

 is curved the strongest branches arise at the highest point of the 



curve. In the cultivation of vines 

 and fruit trees this peculiarity is 

 utilised to produce shorter and 

 weaker shoots (short shoots), 

 which experience has shown are 

 those that bear the flowers. 



4. Mechanical In- 

 fluences. Pressure and 

 traction exert a purely 

 mechanical influence upon 

 growth, and also act as 

 stimuli upon it. External 

 pressure at first retards 

 growth ; it then, however, 

 stimulates the protoplasm 

 and occasions the distension 

 of the elastic cell walls, and 

 frequently also an increase 

 of turgor. As a consequence 



of this increased turgtfT, the 



CO unter-resistailCe to the CX- 

 A j 



ternal pressure is intensified. 

 If the resistance of the body exerting the pressure cannot be overcome, 

 the plasticity of the cell walls renders possible a most intimate contact 

 with it; thus, for instance, roots and root-hairs which penetrate a 

 narrow cavity fill it so completely that they seem to have been poured 

 into it in a fluid state. It would be natural to suppose that the effect 

 of such a tractive force as a pull would accelerate growth in length by 

 aiding and maintaining turgor expansion. But the regulative control 

 exercised by the protoplasm over the processes of growth is such that 

 mechanical strain first acts upon growth to retard it, but then causes 

 an acceleration of even 20 per cent. 



Other actions of mechanical influences as stimuli may be mentioned. Lateral 

 roots arise only from the convex sides of curved roots (Fig. 262), the cause lying 

 probably in the DIFFERENCES OF TENSION between the two sides. The primordia 

 of the haustoria of Cuscuta and the adhesive discs on the tendrils of some species 

 of Parthenocissus are caused to develop by the STIMULUS OF CONTACT. 



FIG. 261. Twigs of Willow : 1, in the -normal position ; 2, 

 in the inverted position growing in a moist chamber. 

 (After VOCHTING.) 



