260 BOTANY PART i 



MOISTURE exerts a twofold influence upon growth. It acts as a 

 stimulus, and also, by diminishing transpiration, increases turgidity. 



Plants in damp situations are usually larger than those grown in dry places, 

 and in fact may differ from them in their whole habit and mode of growth. Direct 

 contact with water seems frequently to exert a special influence upon the external 

 form of plants. Amphibious plants, that is, such as are capable of living both 

 upon land and in water, often assume in water an entirely different form from that 

 which they possess in air. This variation of form is particularly manifested in the 

 leaves, which, so long as they grow in water, are frequently linear and sessile or 

 finely dissected, while in the air their leaf-blades are much broader and provided 

 with petioles (cf. Fig. 35). According to M'CALLUM, Proserpinaca palustris 

 forms dissected leaves even in a saturated atmosphere, while in salt solution it 

 develops leaves with a flat lamina as in the air. The leaf-stalks and internodes also 

 often exhibit a very different form in air and water, and undergo the same abnormal 

 elongation as in darkness. This is especially noticeable in submerged water plants, 

 whose organs must be brought to the surface of the water (young stems and leaf- 

 stalk of Trapa natans, stem of Hippuris, leaf-stalk of Nymphaea, Nuphar, Hy<lr<>- 

 charis). Such plants are enabled by this power of elongating their stems or lent 

 stalks to adapt themselves to the depth of the water, remaining short in shallow 

 water and becoming very long in deep water. 



The great importance of free OXYGEN has already been alluded to in connection 

 with respiration (p. 240). Without gaseous or dissolved oxygen in its immediate 

 environment the growth of a plant entirely ceases, at least in the case of aerobionts. 

 MECHANICAL INFLUENCE. Pressure and traction exert a purely mechanical 

 influence upon growth, and also act at the same time as stimuli upon it. External 

 pressure at first retards growth ; it then, however, acccording to PFEFFEK, stimu- 

 lates the protoplasm and occasions the distension of the elastic cell walls, and 

 frequently also an increase of turgor. As a consequence of this increased turgor, 

 the counter-resistance to the external 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 so completely fill it 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 

 sustaining turgor expansion. But the regulative control exercised by the protoplasm 

 over the processes of growth is such that mechanical strain, as HEGLER has shown, 

 first acts upon growth to retard it (except in the maximum of the grand periods), 

 but then causes an acceleration of even 20 per cent. According to BALL, a 

 constrained position may induce great thickening of the walls on particular sides 

 (e.g. the convex side) of an organ. 



Rarefaction of the air, chemical stimuli, and internal states may exert consider- 

 able influence on growth. In this way the formation and development of the repro- 

 ductive organs, or the assumption of a twining habit may give rise to a striking 

 elongation of the internodes together with a reduction in the size of the leaves. 



According to TOWNSEND, slight wounds accelerate growth, while more serious 

 ones retard it ( 72 ). 



The Internal Development of the Organs 



The internal development of the organs is only completed after 

 they have finished their elongation and attained their ultimate size. 



