364 Mr F. Darwin and Mr R. W. Phillips, [Nov. 23, 



The current is supposed to flow upwards, and in passing from 

 the line ab to cd has to traverse a large number of partitions, 

 formed by the longitudinal walls of the elements in which the 

 current flows. The longer ac is, i.e. the more the saw-cuts over- 

 lap, the greater is the number of such walls and the greater the 

 block in the current. Again, the shorter ab is, the fewer will be 

 the pits through which the current can find its way ; and this 

 rasij account for the facts given in Experiment No. 35, where 

 the amount of diminution of the current is shown to be much 

 greater when ab is 2 cm. than when it is 10 cm. 



It may be urged that all these arguments apply to wood such 

 as that of the Yew, consisting entirely of tracheids, as well as to 

 angiosperm wood, and that therefore we ought to have the same 

 effect in gymnosperms as in angiosperms. 



The tracheids of the Yew are at least 70 or 80 times as long 

 as they are broad, so that in travelling transversely the length of 

 a single tracheid the water current has to traverse 70 cell walls 

 instead of one. So that it is difficult to see why double sawing 

 produces so slight an effect in the Yew. It must however be 

 remembered that in the case of wood consisting altogether of 

 tracheids, a transverse current flows through the same elements 

 as a longitudinal current. It still travels from tracheid to tracheid. 

 But in angiosperm wood the current when forced to travel trans- 

 versely no longer travels in its former elements, since the vessels 

 are destroyed as channels, but must travel chiefly in the thick- 

 walled (what is more important) scantily pitted wood-cells. So 

 that it is unable to travel laterally with any ease unless it has 

 a long space to move transversely in. 



On the whole we think that the difference which probably 

 exists between angiosperms and the Yew or Scotch Fir is to be 

 explained by the fact that after the operation of double sawing the 

 only tissues left are badly fitted for transmission, while in the case 

 of the Yew or Fir no tissue is rendered incapable of transmission. 

 Further experiments would be needed to clear up the point satis- 

 factorily. 



§ 9. Experiments on bending and compressing the tissues of 

 transpiring branches. 



Dufour showed that twigs bent sharply on themselves, so that 

 the apical and basal halves are parallel, do not wither, nevertheless 

 he found that such branches are highly impervious to a stream of 

 water under pressure. Hence, as before, he argues that the tran- 

 spiration-stream does not flow in the cavities of the elements, for 

 if it did so, it would be hindered in the same way as the filtration- 

 stream is blocked by the process of bending. We proceed to show 



