508 



PHYSIOLOGY OF GROWTH. 



stem must always be employed side by side, because different 

 specimens behave very differently. 



It may further be remarked incidentally that, as Vochting has 

 specially shown, light exerts an important influence on the 

 development of roots in willow twigs. If we suspend pieces of 

 willow stems, as described, in glass cylinders, cover one cylinder 

 with a black receiver, leaving the other exposed to diffuse daylight, 

 we find that in the light fewer roots break from the cortex than 

 in the dark, and that the root development is also completed more 

 slowly in the light than in the dark. 



In many plant structures polarity as well marked as that of 

 willow stems is to be observed. I have, e.g., placed potato tubers 

 in a box, in winter, so that they were excluded from the light. 2 

 The apices of several tubers were directed upwards, those of others 

 downwards, but always vigorous shoots developed in abundance 

 only at the morphological apex, i.e. at the 

 end of the potato which is opposed to the 

 former point of attachment to the parent. 

 Under the conditions described the shoots 

 can only obtain the water necessary for 

 their formation, as also the necessary food 

 stuffs, from the tuber (see Fig. 170). 



If we suspend in glass cylinders contain- 

 ing some water pieces of root, e.g. of Ulmus 

 campestris, 100-1 50 mm. long, and 10 mm. 

 in thickness, with their morphological 

 apices directed upwards or downwards, 



Tand then exclude the light, they always 

 r produce adventitious shoots at their mor- 



phological base only. New roots are more 

 rarely produced at the morphological apex ; 

 they do not originate here at all readily. 

 Shoots, as we have seen, form at their morphological apex new 

 shoots; roots, on the other hand, yield shoots at their morpho- 

 logical base. 



Fio. 170. Germinating 

 potato tuber. T, runner. 



1 See Vochting, Ueber Organbildang im PJlanzenreich, 1878. See further 

 Vochting, Ueber Transplants ionen am Pjlanzenkorper, Tubingen, 1892. 



2 See Detmer, Sitzungsber. d. Jenaischen Gesellsch. f. Naturwissensch. und 

 Medicin, 1884, p. 5. 



