Chap. III. OF THE EADICLE OF THE BEAN. 133 



be the most convenient and effectiye. AVe employed 

 at first ordinary thin card, snch as visiting cards, oi 

 bits of very thin glass, and various other objects ; but 

 afterwards sand-paper was chiefly employed, for it was 

 almost as stiff as thin card, and the roughened surface 

 favoured its adhesion. At first we generally used very 

 thick gum-water; and this of course, under the cir- 

 cumstances, never dried in the least ; on the contrary, 

 it sometimes seemed to absorb vapour, so that the bits 

 of card became separated by a layer of fluid from the 

 tip. When there was no such absorption and the card 

 was not displaced, it acted well and caused the radicle 

 to bend to the opposite side. I should state that 

 thick gum-water by itself induces no action. In most 

 cases the bits of card were touched with an extremely 

 small quantity of a solution of shellac in spirits of 

 wine, which had been left to evaporate until it was 

 thick ; it then set hard in a few seconds, and fixed the 

 bits of card well. When small drops of the shellac 

 were placed on the tips without any card, they set into 

 hard little beads, and these acted like any other hard 

 object, causing the radicles to bend to the opposite 

 side. Even extremely minute beads of the shellac 

 occasionally acted in a slight degree, as will hereafter 

 be described. But that it was the cards which chiefly 

 acted in our many trials, was proved by coating one 

 side of the tip ^\ith a little bit of goldbeaters' skin 

 (which by itself hardly acts), and then fixing a bit of 

 card to the skin with shellac which never came into 

 contact with the radicle : nevertheless the radicle bent 

 away from the attached card in the ordinary manner. 



Some preliminary trials were made, presently to 

 be described, by which the proper temperature was 

 determined, and then the following experiments were 

 made. It should be premised that the beans were 



