MO VUMENTS OF PLANTS. 201 



length than the other tissues. Thus in a young internode of the Moun- 

 tain Ash, 60 min. long, the pith, when isolated, elongated 3 mm., while 

 the surrounding parts shortened 1 mm. Close examination of the tissues 

 surrounding the pith shows that they also have developed unequal- 

 ly. Sachs expresses this inequality by the formula, E < C < X < P, 

 which indicates that the epidermis is shorter than the cortex, the 

 cortex shorter than the xylem, and the xylem shorter than the pith. It 

 is at once evident that in such a condition of things the epidermis is 

 elongated by the other tissues ; the cortex is shortened, on the one 

 hand, by the epidermis, and elongated on the other by the xylem and 

 pith ; the xylem is shortened by the cortex and epidermis, and elon- 

 gated by the pith ; while the pith is shortened by the three surround- 

 ing tissues. There is thus a considerable tension in the several tissues, 

 and upon this condition it may be remarked : 



1st. That it produces a rigidity of the steins or other organs in which 

 it occurs. 



3d. That it tends to prevent ordinary torsion ; for the twisting of 

 such a stem must elongate still more the already elongated tissues, 

 while contracting the shortened ones ; on the other hand, there is some 

 tendency to an internal torsion. 



3d. That the exact length of a stem is dependent upon a balancing 

 of the tensions of its tissues. 



There are in many cases tensions whose directions lie at right angles 

 to the foregoing. Thus in the trees of the colder climates the growth 

 of new tissues from the cambium layer produces an outward pressure 

 upon the bark, and an equal inward pressure upon the wood. Even in 

 herbaceous plants similar tensions are often to be observed, the epider- 

 mis being laterally distended by the enclosed tissues. Tensions in this . 

 direction have been denominated transverse tensions, to distinguish 

 them from the others, which may be called longitudinal tensions.* 



* For a full discussion of tensions the student is referred to larger 

 works, such as Sachs' "Lehrbuch," and his " Experimental-Physi- 

 ologic." 



The whole subject of the movements of plants, including heliotro- 

 pism and geotropism, is fully treated by Mr. Darwin in his recent 

 work " The Power of Movement in Plants," New York, 1881. 



