MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 87 1 



tropism is shown by the fact that the revolving nutation takes place more quickly 

 towards the light than away from it. Some tendrils, strikingly those of the Virginian 

 Creeper and Bignonia capreolata^ have the remarkable power of developing broad discs 

 at the end of their branches when they remain in contact for some time with hard 

 bodies, which attach themselves like cupping glasses to rough surfaces, and enable the 

 plant to climb up vertical walls when it finds no slender support round which it can coil. 

 In this case it is obviously necessary that the tendril should turn towards the wall which 

 serves as its support in order to become attached to it, and this is effected by negative 

 heliotropism. which causes the tendril to approach the wall shaded by foliage, where it 

 now performs its revolving movements of nutation — one might almost say its groping 

 movements — creeps along the surface, finds out the crevices and depressions, and 

 developes its adhesive discs. 



Sect. 26. — Movements of growing Leaves and Floral Organs produced 

 by variations of Light and of Temperature ^ In the foregoing paragraphs we 

 have become acquainted with the movements of curvature of growing organs which 

 take place when the external conditions are constant, movements which are pro- 

 duced by the more rapid growth of the one or of the other side of the organ under 

 the action of purely internal causes. These movements were termed * spontaneous 

 nutations.' Amongst the organs endowed with spontaneous nutation we found that 

 tendrils are peculiar in being sensitive to contact on one side and in that the slight 

 pressure of the support induces a more rapid growth of the free surface, and a much 

 less rapid growth of the surface with which it is in contact. Many growing foliage- 

 leaves and floral organs, possessing, Hke tendrils, a bilateral organisation, are stimu- 

 lated to curve by variations of temperature or of the intensity of light, the growth of 

 one side or the other being either accelerated or retarded. 



It is not all growing leaves and flowers that are sensitive to these meteorological 

 influences ; among plants which are very closely allied some do and some do not 

 possess this property. Pfefifer mentions, as examples of sensitive growing leaves, in 

 addition to the very sensitive leaves of Lnpatiens nolitangere, those of Chenopodieae, 

 Atripliceae, Solanese, Mimulus tigrinus, Mirabilis Jalapa^ of species of Silene and 

 Alsine, and of many Compositae; to this list Batalin adds Malva rotundifolia^ (Eno- 

 thera sp., Portulacca oleracea, Linum grandiflorum, Stellaria media, Gnaphalium uligi- 

 nosum, various species of Polygonum, Senecio vulgaris, Sida Napma, Rumeax Hydrola- 

 pathum, IpomcBa purpurea, and Brassica oleracea, and doubtless further investigation 

 will increase the number. The movements of these leaves are not efl'ected by means 

 of special organs, but it is the petiole or the lower part of the lamina of the leaf 

 which curves, under the meteoric influences, upwards or downwards accordingly as 

 the growth in length of the upper or of the lower surface has been accelerated by 

 them. The mode of antagonism of the two sides of the bilateral organ is difi^erent 

 in diff"erent species : in some the leaves are raised at night, as in Chenopodiumy 

 Brassica, Polygonum aviculare, Stellaria, Linum, in others the leaves fall, as in 



^ Pfefler, Physiologische Untersuchungen, Leipzig 1873, and Sitzungsber. der Ges. zur Beford. 

 der ges. Naturwiss. zu Marburg, 1873. — Batalin, Flora, 1873. 



[Pfeffer, Die Periodischen Bewegungen der Blattorgane, 1875. — Darwin, Movements of Plants. 

 — Wiesner, Die heliotropische Erscheinungen, II.] 



In these works full references are given to the older literature of the subject. 



