312 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



time {presentation time), and the bending region (situated a considerable distance 

 back from the tip) must be enlarging. After a period considerably longer than the 

 presentation time {reaction time) bending occurs in the bending region. In clinostat 

 experiments the primary protoplasmic disturbance of stimulation may be started by 

 stationary exposure for a period somewhat longer than the presentation time, and 

 then slow rotation may begin and continue. In such experiments bending occurs 

 after the lapse of the reaction time; the period from the end of the presentation time 

 to the end of the reaction time is called the latent period. 



The hypothesis has been advanced that when the sensitive region first comes to be 

 out of its equilibrium position some heavier cell components {e. g., starch grains) begin to 

 settle through the somewhat viscous cell contents, finally (at the end of the presenta- 

 tion time) coming to be on the physically lower side of each cell. This shifting of 

 starch grains, etc., is supposed to release some form of process, which may be the first 

 of a series or chain of chemical and physical disturbances, the final one of which acts 

 directly on the cells of the distant enlarging (bending) region and produces unequal 

 enlargement on the opposite sides. The latent period is here supposed to be the time 

 necessary for the protoplasmic disturbance to be transmitted from the sensitive to the 

 bending region. The differences in direction between the bendings produced by the 

 same gravitational pull (between positive and negative geotropic bendings, etc.) must 

 be due to internal differences in the bending organs themselves. 



7. Influence of Nutrition on Growth and Configuration. — The nutrition supply, 

 both of organic substances and of salts, exerts great influence on the rate and kind of 

 growth. Especially in lower forms, such as moulds and bacteria, different nutrient 

 media may produce pronounced morphological differences. 



8. Influence of Wounding, Traction, and Pressure on Growth and Configuration. — 

 Wounding of an enlarging tissue may retard or check enlargement, and therefore 

 result in bending. The Darwinian {traumatropic) response of roots is a bending 

 away from an object that wounds the tip, the bending itself occurring in the region of 

 elongation above the tip. Some of the tissue strains in a bending primary root appear 

 to favor the production of branches; the latter more often arise on the convex than on 

 the concave side of a bend, or than on unbent portions of the primary root. Para- 

 sitic insects or fungi often cause striking structural peculiarities in the host plant (e. g., 

 "witches' brooms")- Some dioecious plants bear perfect flowers when infected with 

 the right fungus. Traction (as by thread, pulley and weight) may greatly modify the 

 rate and kind of growth in ordinary plants. External pressure that hinders or checks 

 enlargement (as when a growing root is enclosed in a rigid plaster cast) has marked 

 influence on the maturation of the tissues. 



Pfeffer's experiments showed that the downward pressures exerted by enlarging 

 roots may be very great, as great as 350 grams in a root of Windsor bean, or 200 grams 

 (over 19 atmospheres) per square millimeter of the cross section of the root. 



