sect, u PHYSIOLOGY 237 



water and becoming very long in deep water. The pressure of the water npon the 

 tip of the growing organ, as well as the insufficient supply of oxygen, seem to act 

 upon tlie growth, in this instance, as regulating stimuli. 



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

 with respiration (p. 219). "Without gaseous or dissolved oxygen in its immediate 

 environment the growth of a plant entirely ceases. 



Mechanical Influence. — Pressure and traction exert a purely mechanical influ- 

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

 pressure at first retards growth ; it then, however, according to Pfeffer, 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 grow-th is such that mechanical 

 strain, as Heglee has shown, acts upon growth to retard it (except in the maximum 

 of the grand periods). The elastic resistance and rigidity of cell walls are in- 

 creased by the action of a strain ; such a strain may also induce the formation of 

 collenehyma and sclerenchyma which would not otherwise have been developed. 



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. 

 They are then first enabled to fully exercise their special function. 

 To this end cell cavities usually become more or less fused, and the cell 

 walls thickened, often in a peculiar and characteristic manner (p. 75). 



In the case of plants equipped for a longer duration of life, a 

 growth in thickness follows the growth in length (p. 119). 



Periodicity in Development, Duration of Life, and Continuity of 

 the Embryonic Substance 



The periodically recurring changes in the determinative external 

 influence, especially in light and temperature, occasioned by the alter- 

 nations of day and night and of the seasons, cause corresponding 

 periodical variations in the growth of plants. These variations do 

 not follow passively every change in the condition of the external 

 influences. On the contrary, the internal vital processes of plants so 

 accommodate themselves to a regular periodicity that they continue 

 for a time their customary mode of growth, independently of any 

 external change. The nightly increase of growth, which is especially 

 noticeable after midnight in the curve of growth, and the retardation of 

 growth, specially marked after mid-day, will continue to be exhibited 



