48(3 PHYSIOLOGY OF GROWTH. 



of biological significance that the roots always turn away from the 

 bodies causing the injury, and acting as stimuli. 1 



In contrast with the above, we have root nutations brought 

 about by stimulation not of the root tip, but of the region of the 

 root which is in active growth. The roots then bend, as is the 

 case with tendrils, towards the bodies producing the stimulus. A 

 few pea seedlings, with roots about 2 cm. long, are fixed on pins, 

 and arranged under a bell-glass standing in water, with their 

 roots directed horizontally. Beside each seedling we bring a 

 thin pin, so as to touch the root in its most actively growing 

 region (i.e. about 3 mm. from the tip). The contact acts, as in 

 tendrils, as a stimulus, and since the free side of the root becomes 

 by more active growth convex, the root curves after a time 

 towards the pin. 



Characteristic stimulation effects are brought about, as may 

 here be briefly mentioned, by the bite of insects. The numberless 

 varieties of galls occurring on leaves are produced in this way? 

 and it is hence not without interest, from the physiological point 

 of view, to examine a gall carefully. 



On the leaves of many kinds of willow we often find during the 

 summer galls resembling a small bean, fleshy, and projecting on 

 both sides of the leaf. These are due to the bite of leaf wasps 

 (Nematus Vallisnerii) . The wasps lay their eggs in the tissue of 

 the leaf while it is quite young, and the grub, creeping out of the 

 egg, develops in the gall, which attains a comparatively large 

 size. The stimulus produced by the bite of the insect results in 

 hypertrophy of the leaf tissue, which expresses itself in the 

 formation of the gall. Microscopic examination of thin transverse 

 sections teaches that the Nematus galls consist chiefly of small- 

 celled tissue, the elements of which are approximately isodiametric. 

 In this tissue are enclosed in various places cells elongated in a 

 radial direction. It is also to be remarked that the tissue in the 

 middle of the gall is made up of smaller cells than that at the 

 periphery. 



1 See Darwin, Tlie Power of Movement in Plants, and Wiesner, Bewegungs- 

 vermogen der Pjianzen, p. 139. 



