Dixon and Atkins — Osmotic Pressm-es in Plants. 391 



6. Tlie sheath of wood parenchyma round the vessels functions as a 

 gland to secrete carbohydrates into the rising transpiration stream. 



7. The relation of the medullary rays to these sheaths supports the view 

 that they convey the carbohydrates from the bark to the glandular sheaths. 



8. The presence of large quantities of soluble carbohydrates in the wood- 

 sap of roots is probably responsible for root-pressure and bleeding by pro- 

 ducing an osmotic flow across the root-cortex, which acts as a semi-permeable 

 membrane. 



9. The curves for the concentration of solutes in the stem of the ever- 

 green Ilex, and of the sub-evergreen Ootoneaster, show smaller fluctuations 

 than do those of deciduous trees ; they have two cusps — one about January, 

 and the other about August, in Ilex, and in Februaiy and October in 

 Cotoneaster. It is to be noted that in the case of Ilex the buds expanded 

 during the rise preceding each of these cusps. 



10. The concentration of the carboliydrates is generally greater in the 

 tracheae of the stem than in those of the root, except during the summer. 

 This rule is broken by Ilex where the concentration in the root is the greater 

 throughout the year, except in winter. The electrolytes, liowever, are present 

 as a rule in greater quantity in the root. 



11. In general the vessels function, in times when water is abundant, 

 to convey rapidly solutions of organic and inorganic substances to the 

 leaves. The columns of tracheids may be supposed to afford a permanent 

 channel for the water and salts, and to a less degree for the organic 

 substances. This is never put out of action, even in times of greatest 

 drought. 



Literature. 



(1). Dixon, H. H., and Atkins, W. K.. G. — Osmotic Pressures in Plants: 

 I. Methods of Extracting Sap from Plant Organs. Proc. Roy. 

 Dub. Soc, 1913, vol. xiii. (n.s.), p. 422, and Notes from the 

 Botanical School of Trinity College, Dublin, 1913, vol. ii, p. 152. 



(2). Dtxon, H. H., and Marshall, B. S. — A Quantitative Examination of 

 the Elements of the Wood of Trees in relation to the Supposed 

 Function of the Cells in the Ascent of Sap. Proc. iioy. Dub. Soc, 

 1915, vol. siv (n.s.), p. 358. 



(y). Fischer, A. — Glycose als Reservestoff der Laubliolzer. Bot. Ztg., 

 1888, s. 405. 



