DARWIN ON THE MOVEMENTS OF PLANTS. 503 



or -001 of an inch, and then a slow retreat for part of the distance. 

 No such movement could be detected with a two-inch object-glass, 

 in the case of Drosera, and how far it is general is not known. Mr. 

 Darwin says : " The whole hypocotyl (stem of a cotyledon) of a cab- 

 bao-e or the whole leaf of a Dionoea * could not ierk forward unless a 

 very large number of cells on one side were simultaneously affected. 

 Are we to suppose that these cells steadily become more and more 

 turgescent on one side, until the part suddenly yields and bends, 

 inducing what may be called a microscopically minute earthquake in 

 the plant ; or do the cells on one side suddenly become turgescent in 

 an intermittent manner each forward movement thus caused being 

 opposed by the elasticity of the tissues ? " 



Mr. Darwin has shown the importance of this ever-present move- 

 ment in successive chapters upon modified circumnutation. By this 

 phrase he means that pressure and other irritants, light and gravita- 

 tion, do not directly cause movement ; they only modify the sponta- 

 neous changes in the turgescence of the cells, which are always in 

 progress, and of which circumnutation is a universal consequence. He 

 thinks that, in the case of seedlings, ordinary or unmodified circumnu- 

 tation is clearly of service, directly or indirectly ; but, in the later 

 stages of growth, it is from various modifications of this constant 

 motion that the plant derives benefit. 



More than half the volume is given to the modifications of cir- 

 cumnutation by epinasty and hyponasty ; f by nyctitropic or sleep- 

 movements ; and by the influence of light and of gravitation : but we 

 can only glance at one of these, which is popularly styled the sleep of 

 leaves, although there is probably no real analogy between the sleep 

 of animals and that of plants. 



Le 



III. 



Fig. 5. Trtfolium strictum : Diurnal and Noctttrnal Positions of the Two Cottxedons 

 AND OF THE FiRST Leaf : I. Seedling viewed obliquely from above, during the day: Re, right 

 cotyledon ; Lc, left cotyledon ; F, first true leaf. II. A rather younger seedling, viewed at 

 night : Re, right cotyledon raised, but its position not otherwise changed ; Z,c, left cotyledon 

 raised and laterally twisted ; F, first leaf raised and twisted so as to face the left twisted coty- 

 ledon. III. Same seedling viewed at night from the opposite side. The back of the first leaf, 

 F, is here shown instead of the front, as in II. 



Nyctitropic {night-turniyig) is the word used by Darwin to de- 

 scribe the sleep of leaves and occasionally of flowers, but^ as flowers 

 are affected chiefly by changes of temperature instead of light, their 



* In these instances the jerking motions were very remarkable. 



\ The alternately more rapid growth of the upper and under surfaces of organs. 



