TISSUE-TENSIONS IN THE MOTILE ORGANS. 63 1 



Strands marked G and g, in Fig. C, here come close together, leaving a channel 

 above which is filled up with the pith m. As regards the finer anatomical structure 

 of the motile organ, reference may be made to Fig. 371 below, which represents the 

 transverse section of the motile organ of Oxalis, the relations in both cases agreeing 

 in all essential points. The epidermis of the motile organ is relatively insignificant 

 and not strongly cuticularised, but furnished with hairs which, however, do not interest 

 us further. 



A property of the motile organ of the Bean which is important for our 

 purpose, and which also is again met with in all other similar motile organs, consists 

 in the tension of the tissues ; for the movements are practically nothing but alterations 

 in the tissue-tension as a whole, and their relative sizes on the upper and lower side 

 of the organ. In this is conspicuous above all the extraordinary magnitude of this 

 tension, which depends on the one hand on the strong turgescence of the irritable 

 parenchyma, and on the other hand on the toughness and elasticity of the non-lignified 

 strand. By means of the one the thick parenchymatous envelope tends to be 

 forcibly extended, and tihis is prevented by means of the other. The obvious 

 consequence is, as explained in Lecture XIII, that the entire motile organ, although 

 consisting of eminently succulent tissue, nevertheless possesses a very high degree of 

 rigidity, which is also absolutely necessary in order that it may support the 



Fig. 369. — Transverse and longitudinal plates front the motile organs of tbe leaf of the Bean, lying in water in order to show the 



changes in turgescence- 



weight of the leaf-stalk and leaves, the centre of rriotion of which lies just in 

 this organ. If, by the decrease of turgescence in the parenchyma, the organ 

 became limp, the partial loss of rigidity would result in the attached leaf being 

 depressed by its own weight: with the changes produced in the parenchyma 

 by variations in the light, alterations in the turgescence are, as a matter of fact, 

 connected. 



Since it is absolutely impossible to obtain a clear idea of the facts here under 

 consideration unless this point is kept in view, I may attempt to illustrate the above- 

 mentioned properties of the motile organ still further, by means of Fig. 369. 

 A represents a longitudinal section (or better, a longitudinal plate) of the organ, 

 cut off transversely above and below. It is noticed how the two halves ss oi 

 the parenchyma swell and project above and below, because the axial strand g 

 is too short for them. In B the right half of the swelling parenchyma s has 

 been separated from the axial strand by a longitudinal section, and it at once becomes 

 curved in the manner shown at j on the right. By means of a second section, the left half 

 of the swelling tissue has been first bisected, and then the inner part separated from 

 the strand ; this became curved concave towards the strand, while the external half 

 became curved concave outwards. From B then, it is clear that in the parenchy- 

 matous envelope of the organ the external layers are likewise in a state of tension 

 towards the internal layers, but so that the entire mass of tissue tends to become 



