547 



vessels is inccnspicuous where the intermittent strains are but slight ; 

 but it is conspicuous at those joints on which lateral oscillations of 

 the attached branches throw great extensions and compressions of 

 tissue. Throughout the Cactacece we find varied examples of the 

 alleged relation. Mammillaria furnishes a very marked one. The 

 substance of one of these globular masses, resting on the ground, 

 admits of no bending from side to side ; and accordingly its large 

 bundles of spiral and annular vessels, or partially-united cells, have 

 very feebly-marked sheaths not at all thickened. In such types as 

 Cereiis and Opuntia we see, as in the Euphorbias, that where little 

 stress falls on the vessels, little deposit takes place around them ; 

 while there is much deposit where there is much stress. Here let me 

 add a confirmation obtained since writing the above. After observ- 

 ing among the Cactuses the very manifest relation between strain 

 and the formation of wood, I inquired of Mr. Croucher, the intelli- 

 gent foreman of the Cactus-house at Kew, whether he found this 

 relation a constant one. He replied that he did, and that he had 

 frequently tested it by artificially subjecting parts of them to strains. 

 Neglecting at the time to inquire how he had done this, it afterwards 

 occurred to me that if he had so done it as to cause constant strains, 

 the observed result would not tell in favour of the foregoing inter- 

 pretation. Subsequently, however, I learned that he had produced 

 the strains by placing the plants in inclined attitudes — a method 

 which, by permitting oscillations of the strained joints, allowed the 

 strains to intermit. And then, making the proof conclusive, Mr. 

 Croucher volunteered the statement that where he had produced 

 constant strains by tying, no formation of wood took place. 



Aberrant growths of another class display the same relations of 

 phenomena. Take first the underground stems, such as the Potato 

 and the Artichoke. The vessels which run through these, slowly 

 take up the dye without letting it pass to any considerable extent 

 into the surrounding tissues.* Only after an interval of many hours 

 does the prosenchyma become stained in some places. Here, as 

 before, an absence of rapid exudation accompanies an absence of 

 woody deposit ; and both these go along with the absence of inter- 

 mittent strains. Take again the fleshy roots. The Turnip, the 

 Carrot, and the Beetroot, have vessels that retain very persistently 

 the coloured liquids they take up. And differing in this, as these 

 roots do, from ordinary roots, we see that they also differ from them 

 In not being woody, and in not being appreciably subject to the 



* Those who repeat these experiments must be prepared for great irregu- 

 larities in the rates of absorption. Succulent structures in general absorb 

 touch more slowly than others, and sometimes will scarcely take up the dye 

 at all. The differences between different structures, and the same structure 

 at different times, probably depend on the degrees in which the tissues are 

 charged with liquid and the rates at which the}' are losing it by evaporation.. 



