388 Scientific Proceedings^ Rot/al Dublin Society. 



Starch was present iu large quantities in the wood at the time these 

 determinations were made. The cells of tlie medullary rays, tlie last few 

 layers of elements formed in eacli year-ring and the first layer of the next, 

 and the elements in contact with the vessels were densely crowded with 

 starcli grains. The sheath of starch-containing elements round the vessels 

 was continuous, and often many-layered, in the root and in the stem at the 

 ground-level. Higher up only some of the elements in contact with the 

 vessels contained stareli, but those which did so were densely packed. Also 

 it was noticed that at the higher levels, the first layer of the spring wood 

 was without starch, except where it was in contact with vessels. G-enerally 

 there appeared fewer starch-containing elements at the higher levels. In 

 the root the number of starch-containing cells is still further augmented by 

 the fact that the vessels are much more numerous, hence the number of 

 elements constituting their starch-bearing sheaths is more considerable. A 

 quantitative estimation of the cross-section of the various elements of the wood 

 in some trees has been given in a previous paper (2). 



FimclioH of the. Living Elements of the Wood. 



Examination of sections for the estimation of the starch content of the 

 wood cannot fail to force on one the remarkably regular and close connexion 

 existing between the starch-containing elements and the vessels. Further, 

 when we take into account that the transference of carbohydrates can no 

 longer be regarded as an occasional and accessory function of the vessels, 

 but is certainly a continual and principal function, tlie starch layer round 

 each of them becomes evidently a glandular sheath to the vessel for the 

 secretion' into it of the carbohydrates to be transmitted upwards. The 

 location of starch in the elements on the borders of the year-rings is clearly 

 connected with the sudden transmission upwards of immense quantities of 

 carbohydrates in the spring. The depletion of the glandular layer of the 

 spring vessels will be made good from the stores massed close by in the outer 

 margin of the year-ring. 



We may imagine these carbohydrate glands forming a sheath round the 

 vessels to act somewhat as follows: — In spring their stored starch is rapidly 

 brought into solution, and the resulting sugars secreted into the vessels. The 

 concentrated solution in the tracheae acting osmotically through the semi- 

 permeable membrane formed by the outer tissues of the root determines a 

 flow of water from the soil to the traclieae, and the resulting hydrostatic 



1 It is quite possible that simple diffusion of sugars from the living cells into the tracheids may 

 be quantitatively sufficient to explain their presence in the latter, since protoplasm is not rigidly 

 impermeable to these substances, but often allows their penetration at a relatively great rate. 



