MOVEMENTS OF IRRITATION. 503 



mately connected with the contact stimulus which led to the 

 formation of the attachment balls. If, viz., the Ampelopsis ten- 

 drils do not attach themselves, the spiral turns are not produced ; 

 the tendrils become withered, even in the course of one or two 

 weeks, and fall off. The contact stimulus, however, leading to 

 formation of attachment balls at the ends of the tendril branches, 

 calls forth not only the formation of the spiral windings referred 

 to, but, owing to transmission of stimuli, it leads to still further 

 changes in the freely extended part of the tendril. 



If we examine a thin transverse section of a non-attached ten- 

 dril branch of Amelopsis, we perceive a large-celled pith, which 

 is surrounded by a vascular bundle circle. The communication 

 between the pith and the green cortex is effected by wide medul- 

 lary rays, and in the periphery of the cortex, close under the 

 epidermis, is present collenchyma. When the tendrils have 

 attached themselves, their structure undergoes essential changes, 

 simultaneously with the formation of the spiral turns. In the 

 medullary rays interfascicular cambium appears ; the wood of the 

 vascular bundles enlarges considerably, till finally a closed ring 

 of wood is produced, whereby the tendrils gain very considerably 

 in firmness and power of resistance, and now become of real 

 service to the plant. 1 



1 See Darwin, The Movements and Habits of Climbing Plants, and v. Len- 

 gerken, Botan. Zeitung, 1885, Nos. 22-26. 



IV. DORSIVENTRALITY, POLARITY, AND ANISOTROPY 



IN PLANT ORGANS, AND PHENOMENA OF 



CORRELATION. 



192. Dorsiventrality. 



Many plant organs, especially plagiotropic ones (e.g. many 

 foliage leaves), are decidedly dorsiventral in construction. Some 

 stem structures are characterised by dorsiventrality, morpho- 

 logical or physiological, and in some of these even the cause of 

 the dorsiventrality is known, and it can hence be induced in them. 

 We will make a few observations in order to gain further infor- 

 mation on this interesting subject. 



We sow some seeds of Tropseolum majus in summer in good 



