256 PART III. — VEGETABLE PHYSIOLOGY. 



are apheliotropic, and by virtue of this property, and since walls ? 

 tree-trunks, etc., are nearly always less strongly illuminated than 

 the sky, they bend toward such surfaces. If they are able to 

 reach them, the sensitive tips of the branches are irritated by 

 the contact, and they enlarge, become flattened into sucker-like 

 discs, which, by means of a cement they secrete, become glued 

 to the surface, affording them a secure hold upon it. The 

 tendrils and their branches then coil into spirals, and acquire 

 great firmness and elasticity, in the same manner as those of the 

 Passion flower above described. The plant is thus enabled to 

 climb over perpendicular walls of rock, the sides of buildings, etc., 

 objects to which most tendril- and leaf-climbers are unable to 

 cling. A portion of Ampelopsis Veitchii is shown in Fig. 18, 

 Part I. 



Not less wonderful is the sensitiveness of young roots, by 

 reason of which they are able, during their progress through the 

 soil, to avoid obstacles or turn aside from their course to reach 

 supplies of moisture. 



Experiment proves that plants, like animals, may have their 

 sensitiveness impaired or destroyed by exposure to anaesthetics 

 like ether, chloroform, etc. 



If the Sensitive-plant be placed in a bell-jar under which a 

 little chloroform is permitted to evaporate, its leaves very soon 

 cease to respond to the touch ; if the exposure be long continued, 

 it fails to recover sensitiveness, and dies ; but if it be of short 

 duration, it soon comes to itself and the possession of its normal 

 powers. This affords another proof that irritability in the plant 

 is essentially the same thing as irritability in the animal. 



How irritant impulses are communicated from one part of an 

 organ to another in the plant, is not yet well understood. Plants, 

 of course, do not possess nerves by which such impulses can be 

 conveyed. They must travel from cell to cell, and it is difficult 

 to understand how this can be, unless the living matter is con- 

 tinuous from cell to tell, through the cell-walls. 



By using sulphuric acid to dissolve away the cell-walls, the 

 protoplasm has been found, in some cases, to be thus connected 

 by very delicate threads. It is only reasonable to suppose that 

 this continuity is not exceptional, but that it exists in all organs 

 capable of transmitting an irritant impulse from one part to 

 another. 



