57° 



LECTURE XX XII J. 



tissue, the more energetic the growth tends to be. On the other hand, howeverj 

 it is of importance to note that the tissue-tension itself is called into being by differ- 

 ences in growth, especially differences in the lengthening of thp various layers of tissue. 

 It begins with their histological differentiation behind the growing-point, and the 

 further this differentiation of epidermis, vascular bundles, strands of sclerenchyma, 

 and parenchyma proceeds, the more pronounced also, up to a certain point, is the 

 of tension, or, put better, the differences in length of the layers of tissue which make 

 their appearance when the layers are separated. But when the vascular bundles and 

 layers of sclerenchyma have become lignified, and the growth in length of the 

 whole organ at this place therewith concluded, the tissue-tension may still persist, 

 though on the separation of the layers the difference in length is not so great; for 

 it is obvious that a very strong tension may be existing in the tissues when only 

 very small or even unmeasurable elongations and shortenings occur on the separa- 

 tion of the layers, for this only demonstrates that the extensibility of the tissues in a state 

 of tension towards one another has diminished. We must not therefore regard the 

 changes in dimension on the separation of the layers of tissue forthwith and generally 

 as the immediate expression of the force of tension ; that would only be possible 

 if the extensibility remained the same at different ages, which is not the case. 

 In spite of this objection, very important for the speciaUst, it is nevertheless 

 necessary for the understanding of the mechanical properties of the growing parts of 

 plants to return once more to the most important phenomena of the tension of 

 tissues S even if only to afford the reader a general idea. For this purpose I 

 may take the following statements from my ^Handbuch der Experimental Physiohgie ! 

 {1865). In the first place a portion of the shoot-axis under consideration was 

 measured, then the tissue-layers mentioned were cut out, and these also measured. In 

 the following tables the original length of the part is regarded as 100, the shortenings 

 being denoted as negative and the lengthenings as positive numbers per cent. 



Similar shortenings of the external layers of tissue, and lengthenings of the 



' A more detailed discussion of the idea of the intensity of the strain in tissue-tension, and above 

 all as to the fact that the mere changes of length of the separated strips of tissue only present a 

 measure for the prevailing forces if the extensibility of the compared tissues remains unaltered, as 

 well as on other considerations appertaining here, are to be found in my 'Lehrbuch^ Aufl. IV, 1874, 

 p. 763. There also are to be found numerous examples of the behaviour of the separated strips of 

 tissue of growing parts of shoots. 



