THE CONDUCTION OF WATER. II 6^ 



into one long cavity by absorption of the transverse partitions. In general, 

 tracheids are shorter and narrower than tracheae. The following table fur- 

 nishes us with data with regard to the dimensions of certain vessels : — 



When we remember that the water lost by the branch in the process of 

 transpiration streams along the vessels, much in the same way as it does in 

 the glass tube which forms part of Askenasy's apparatus, it will be at once 

 apparent to us that the tracheae are much better fitted for water transport 

 than the tracheids. In the first place, they are wider than the tracheids, 

 and a well-known law in physics tells us that the amounts of fluid which pass 

 through two tubes under the same pressure are proportional to the fourth power 

 of the radius. In the second place, the vessels are only rarely interrupted by 

 transverse walls, each transverse partition acting as an obstacle to the movement 

 of water. If the resistance to water conduction in the transporting conduits be 

 alone considered, it is obvious that the longest and widest vessels would be 

 the best fitted for the purpose. Since, however, we find that in the majority 

 of plants narrow and short tracheids occur in addition to long and broad 

 tracheae, we are compelled to conclude that this difference in the form of the 

 conducting elements corresponds to a difference in function ; wherein the 

 division of labour between the two elements consists, however, it is impossible 

 to say. It is known only that, to a certain extent, the tracheids may take the 

 place of the tracheae, a conclusion which may be arrived at from a knowledge 

 of the fact that the wood of many trees (e. g. Coniferae, Drimys, Trochoden- 

 dron, &c.) consists entirely of tracheids, and this may be experimentally 

 determined from deep incisions made right into the pith. The numerous experi- 

 ments on this subject were first correctly interpreted by Strasburger (1891). 

 If several incisions, at suitable distances apart, laterally and vertically, be 

 made in wood which contains only tracheae (e. g. Ficus), the wood is rendered 

 incapable of performing its functions, because all the conducting channels are 

 interrupted. When tracheids occur as well as tracheae, or when tracheids only 

 are present, the incisions do less damage, because, where the interruptions occur, 

 a lateral conduction through the tracheids is effected. This is especially evident 

 if solutions of colouring matters be used ; the tortuous path taken by the water 

 in its ascent is shown by the coloured fluid, while normally it may be demon- 

 strated that the water follows a straight course by means of the same method. 



In addition to a vertical ascent a lateral passage of water in the wood is also 

 possible. Obviously in the case of lateral movement a far greater number of 

 walls must be passed through than in the case of longitudinal movement, 

 whether these be the walls of tracheae or tracheids. But, as we have stated 

 above, the walls present a certain amount of resistance to the movement. 

 Before going more closely into this matter we must study the structure of the 

 wall of the vessel ; a subject which is of interest from other points of view also. 



The first thing noticeable with regard to the wall is its peculiar sculpturing. 

 There are no vessels whose walls are tmiformly thickened for any great dis- 



F 2 



