200 BOTANY. 



When such rotating stems come in contact with an up- 

 right object they continue their rotation, and in this way 

 come to twine around it. The plants mentioned ahove af- 

 ford common examples of twining. In the case of tendrils 

 nutations also occur ; but after coming in contact with any 

 object there is a very unequal growth of the two sides, that 

 in contact with the object growing very slowly, as compared 

 with the rapidity of growth of the outer side. Thus De 

 Vries found that in the tendrils of the pumpkin twined 

 around an object 1.2 mm. in diameter the ratio of the 

 growth of the inner side to that of the outer was as 1 to 14. 

 This inequality of growth is due to a retardation of growth 

 upon the inner side and an acceleration upon the outer. In 

 some cases there appears to be an actual contraction of the 

 inner side. 



263. Movements of Torsion. In many cases in the 

 higher plants the stems or other organs become twisted upon 

 their axes. Even in the lower plants this is not uncommon 

 e.g., in Nitella, the pedicels of mosses, etc. This twisting 

 appears in many cases to be due to a peculiar inequality in 

 the growth of the tissues. Thus if the outer layers of cells- 

 grow in length more rapidly than the inner ones, the stem 

 will become twisted upon its axis, and the greater the ine- 

 quality in growth of the inner and outer layers, the greater 

 the torsion. In some cases torsion arises in a much simpler 

 way, by the twisting due to the unequal distribution of the 

 weight of certain organs, as in some prostrate plants, where 

 the weight of the leaves and the advancing and obliquely 

 ascending growing extremity of the stem produce torsions 

 which become permanent by the hardening of the tissues. 

 Likewise torsions may arise on account of the heliotropism or 

 geotropism of an organ itself, or of organs connected with it. 



It may be in place here to direct attention to the fact that inequali- 

 ties in the growth of the tissues of plants are of common occurrence. 

 They are, however, for the most part of such a nature as to prevent 

 torsions of the stem, giving it, on the contrary, a rigidity which en- 

 ables it to stand erect. If the pith of a growing stem of a Dicotyledon 

 be isolated from the surrounding tissues, the former elongates, while 

 the latter contracts, showing that the pith has grown more rapidly in 



