DIFFERENTIATION OF EMBRYONIC TISSUES 3 1 



with the structure of a mature tendril which has not clasped a 

 support. 



45. Influence of Stretching Forces. Compare the structure of 

 the basal portion of a tendril which has been fastened to a support 

 and borne the weight of a stem with the mature organ which had 

 no stress of this character. 



. A most notable demonstration of the influence of such force 

 may be made if one cucurbitaceous vine is allowed to trail along 

 the ground which will support the weight of the fruits, while a 

 second is trained to a trellis, and the petioles allowed to carry the 

 weight of the large fruits. This effect will extend to the portions 

 of the vine affected by the weight. 1 



46. Differentiation of Embryonic Tissues under Compression, 

 The influence of compression upon the development of tissues 

 may be found by enclosing growing stems in rigid casts of plaster 

 of Paris. Select a vigorous specimen of Vicia, Pisum or Phase- 

 olus, and a cork, which is about five times the diameter of the 

 stem. Bore a hole longitudinally through the cork and split the 

 cork in halves and place the halves together, enclosing the stem 

 in such manner as to fit it tightly. Fasten the halves of the 

 cork together in place by driving pins through them. The cork 

 should enclose the median portion of an internode. Now curve 

 a tough piece of manila paper around the cork and fasten it with 

 pins in such manner that it forms a cylinder with the cork as the 

 bottom. Fasten the edges together with pins. Fill a small 

 evaporating dish half full of plaster of Paris and add water to it 

 slowly, stirring carefully until a creamy paste is formed that can 

 be poured easily. Quickly fill the paper cylinder with the mix- 

 ture and support it in the proper position until the cast hardens, 

 which will need from 30 to 60 minutes. Paper and cork may 

 now be removed. The rigidity of the cast will prevent increase 

 in thickness of the stem and change the stresses among the tis- 

 sues. Ten days later cut away the cast by making two longitu- 



1 Pieters, A. J. The influence of fruit-bearing on the development of mechanical 

 tissues in some fruit trees. Annals of Botany, 10 : 511. 1896. 



