concerning Wood and Lignified cell-walls. 5 



that either resin or tannin is at once suggested ; for instance, the 

 Sycamore is "very active," while the Beech is only "active," and 

 the Alder and Elm are but " slightly active." 



It appears to me, therefore, that while the evidence is in favour 

 of some substance emanating from the cell-walls being the active 

 agent, this substance is either not yet recognised, or it differs in 

 different cases. My reason for agreeing so readily with the con- 

 clusion that it is some substance — perhaps, but not certainly, 

 volatile — is this. Russell found that " a glass or mica screen of 

 one-thousandth of an inch in thickness entirely protects the plate 

 from being acted on"; and he concludes from this that it is not 

 a case of radio-activity. 



Whether his conclusion be sound or not, the efficacy of such 

 a screen — and the fact that a screen of porous substance, or of 

 gelatine, or of wood does not prevent the action — points to some 

 such conclusion as the above. 



Having confirmed Russell's results, I proceeded to the enquiry: 

 (1) Can we obtain similar photographs with thin sections of wood? 

 For, if so, it might be possible to obtain micro-photographs and 

 trace the action — by its relative sharpness — to the solid lignified 

 walls or to the cavities. (2) Can we, by streaking such sections 

 with solutions of substances to be tested — e.g. Tannin, Tannic 

 Acid, Gallic Acid, Xylol, Turpenes, Oils, or any other bodies 

 derived from wood — obtain local intensification of the results ? 

 In this way it might be possible to run down the active bodies, 

 and compare them with the various woods in their natural 

 conditions. 



The first question was soon answered. The results of all the 

 experiments it is proposed to discuss here are summarised in the 

 following Table, in which the experimental running number, 

 the species of wood employed in thin sections, the condition of 

 the wood-section used, and any treatment to which it was sub- 

 jected before placing it on the plate in the dark-room and 

 wrapping it up, are given in the first four columns. 



The time during which the contact with the plate continued is 

 given in column v.; and the temperature during the period of 

 contact in column VI. 



In the last column I have summarised the results in a few 

 words, sufficient to show the principal features of any figures 

 obtained. 



Analysis of this table shows that in most cases a considerable 

 incubation-period is necessary to obtain any picture beyond a 

 " mere ghost," so faint that one can only say that some " activity " 

 has been displayed. 



Whether this is because my thin sections contain very little of 

 the active substance in the small amount of cell-wall present (as 



