TENDRILS 



565 



time when the tendril first becomes irritable, the stereome elements arc 

 still in process of development. The small vascular bundles are situ- 

 ated just outside the stereome cylinder, on its abaxial side. The upper 

 epidermis is separated from the mechanical tissue by a few layers of 

 ordinary parenchyma; on the abaxial side, the space between the 

 epidermis (or sensory epithelium) and the vascular strands is occupied 

 by a bulky motor-tissue, consisting of elongated cells which contain 



B 



Fig. 230. 



A. T.S. through a tendril of Urvillea jcrrugtnea ; o, upper (convex), v., lower 

 (eoncave) side ; m, stereome ; h, hadromc-strands of the vascular bundles ; e, collen- 

 chyma-strands ; b, motor-tissue ; s, tubular excretory sacs. B. Motor-tissue in T.S., 

 more highly magnified. 



no chlorophyll except in the immediate vicinity of the surface. The 

 walls of the motor-cells are thickened in a peculiar manner (Fig. 

 230 b). The middle lamella is particularly well developed along the 

 edges of the cells, and seems to possess very considerable powers of 

 imbibition ; the secondary thickening layers are highly refractive, and 

 assume a blue coloration with chlor-zinc-iodine, thus contrasting sharply 

 with the middle lamella, which remains unstained. As regards its general 

 structure, therefore, the motor-tissue closely resembles that of many 

 irritable stamens. That this tissue is really responsible for the curva- 

 ture of the organ, may be inferred straight away from the fact, that it 

 occupies the great bulk of the cross-section on the irritable and con- 

 tractile side of the tendrils. There is, in any case, no other tissue 



