IRRITABILITY. 41 1 



surface of the root which is in contact with the solid body 

 being less rapid than that of the opposite free surface ; 

 similarly, the aerial roots of Aroids and Orchids become 

 closely applied to solid bodies, and delicate filaments, such as 

 the hyphae of Fungi and pollen-tubes behave in precisely the 

 same manner. The softer and the more yielding the tissue of 

 the organ, the more easily will it be thus affected by pressure ; 

 for instance, the pileus of Agarics growing in woods often 

 grows over and more or less completely encloses leaves, pieces 

 of stick, etc., the slight weight of these objects sufficing to 

 arrest the growth of that portion of the pileus upon which 

 they were lying. Another illustration is afforded by the effect 

 of the mutual pressure of organs arising close together in 

 modifying their individual form and relative arrangement. 

 Schwendener has drawn attention to this in the case of leaves, 

 and has been able to arrive at a simple mechanical explana- 

 tion of the phenomena of phyllotaxy. 



But external pressure has, in many cases, another and 

 quite opposite effect upon growth. In these cases it promotes 

 the growth of the organ, it acts as a stimulus to it. For 

 example, von Mohl has shewn that the haustoria of Cuscuta 

 and Cassytha are only formed when there is continuous con- 

 tact with a solid body ; and similarly, as Darwin and Pfeffer 

 have found, the adhesive discs on the tendrils of the Virginian 

 Creeper are only formed under the same circumstances. In 

 some plants these discs are formed independently of such 

 contact : this has been observed in Ampelopsis Veitchii 

 (MNab), in Haplolophium (F. Miiller), and in Zanonia 

 macrocarpia (Treub) : but it appears that the size of the discs 

 is very much increased by contact. Again, Darwin has 

 pointed out that tendrils and the petioles of leaf-climbers 

 become much thicker, when they have come into contact with 

 a support, than they were before, and Treub has drawn atten- 

 tion to the occurrence of precisely the same thing in the 

 hooks of certain hook-climbers. 



The accompanying figures of sections of a free and of an attached 

 petiole of Solatium jasminoides give some idea of the effect of pressure in 

 stimulating to growth. 



