upon the Cause of Vegetable Divergence. 181 



has proposed a " much simpler and equally satisfactory expla- 

 nation of the phenomenon P From his opinion in this respect, 

 however, I must beg leave to dissent in toto ; but, lest I should 

 be accused of misconstruing the editor's meaning by giving 

 another version of his observations, I shall copy that portion 

 of them which he seems to think most conclusive against the 

 theory I ventured to propose ; omitting only his figure of, and 

 reference to, a longitudinal section of an exogenous stem : — 

 " The plants found to exhibit it (i. e. divergence) are pos- 

 sessed of a vascular, fibrous, and a vesicular tissue. The latter 

 occupies the central parts exclusively ; while the woody fibres 

 occur towards the exterior, as do the vessels properly so called. 

 Now, the woody fibres not being extensive, and the cellules at 

 a (i.e. the centre of the stem) being dilatable, it will neces- 

 sarily follow that, when the section of the stem is made, the 

 cellules being filled with fluid or vapour, which readily passes 

 from one to the other, the larger inner cellules, in consequence 

 of the pressure caused by the unyielding cuticle and herbaceous 

 tissue which compressed the interior when the stem was entire, 

 being now allowed to expand to the full size, or in some de- 

 gree ; while the outer cellules are bound together by the in- 

 extensive woody fibres, the curvature outwards of the section 

 is a necessary result so long as moisture is supplied by the 

 unslit part of the stem, or communicated from without, this 

 fluid ascending on the principle of endosmose. When, as in 

 experiment 2., the slit stem is immersed in water, the diver- 

 gence is increased on the same principle ; but when, as in 

 experiment 3., a denser fluid is substituted, exosmose takes 

 place in the vesicular tissue, in consequence of which the 

 pressure is removed from the larger vesicles, which, by their 

 elasticity, regain their original size. In the plant (experiment 

 4.) whose lower parts were destroyed by poison, the upper not 

 having received a supply of fluid to replace the quantity 

 evaporated in twenty-four hours, no divergence could ensue 

 on slitting it; but, as in experiment 5., should it be filled with 

 fluid, it diverges as explained above. For the reasons stated, a 

 dried piece of stem (experiment 6.) can, of course, undergo no 

 change ; but, if restored to its natural state, as in experiment 7. 9 

 it exhibits the usual phenomenon. In accordance with the 

 explanation is the fact, that woody stems, when slit, do not 

 diverge, because all their parts are equally bound together 

 by firm longitudinal fibres, as well as that of purely cellular 

 plants, such as Fucus palmatus, exhibiting no divergence." 

 [Edin. Journ. Nat. Hist., No. xviii. p. 87.) 



After referring to my own paper, I must confess that I was 

 completely foiled in attempting to discover anything original 



