310 



NA TURE 



[July 29, 1897 



taneously occupied by a transpiration current ; parts of it may 

 be so occupied, while parts of it are filled with air, and do not 

 function as water-ways. This is a valuable contribution to 

 knowledge, and to the adherents of the new theory it is priceless ; 

 the very existence of their hypothesis may depend on it. 



Strasburger's statements and reasoning are by no means 

 accepted by everyone ; for instance, Schvvendener refuses to take 

 them seriously {K. Preiiss. Akad., 1892, p. 931). 



Strasburger has microscopically examined the condition of the 

 tracheals as regards air.^ He found in the spruce fir in July 

 " almost no air bubbles " in the wood of the current year, but 

 air in considerable quantity in four-year-old wood. In the same 

 month Pimis Salzmanni {Laricio) showed scattered bubbles in 

 the spring wood of last year, and more in the autumn wood. In 

 a larch there were only very occasional bubbles in the two last 

 years' wood. In the .silver fir the current year's wood was prac- 

 tically free from air : the air increased in the inner rings. I'stiga 

 canadensis had no air in this year's wood, only a little in last 

 year's, and an increasing quantity in the older rings, the fifth 

 being very rich in air. In February, Pinus strokes had hardly 

 any air in this year's wood, and the silver fir was all but free 

 from it in the youngest ring. Robinia in July had the youngest 

 wood almost air-free. Ficus elastica and spuria, various 

 Acacias, and willows gave vessels not entirely free from air, 

 but nearly so. He concludes {loc. cit. p. 688) that the path of 

 the transpiration current is not absolutely free from air. The 

 younger wood, which especially functions as the water-carrier, 

 is the most free. 



•Dixon and Joly quote Strasburger's results, which they con- 

 sider sufficiently favourable to their views. They rely, in addi- 

 tion, on the impermeability of wet cell-walls to air, isolating the 

 conduits in which air has appeared ; and on the possibility that 

 the air may be redissolved under root-pressure {Phil. Trans. 

 p. 572), an idea well worth testing. 



I think Strasburger's facts are not so favourable to their 

 theory as these authors believe ; in the same way it seems 

 to me that Askenasy is rash in saying "^ that the tracheals 

 in many cases contain continuous columns of water. It is 

 true that this statement does not affect the validity of his 

 general argument, since he faces the undoubted occurrence of 

 air bubbles in many cases. This is undoubtedly necessary, and, 

 fortunately, we can once more turn to the Leilungsbahnen. 

 Strasburger states that he has seen water creep past the air 

 bubbles {Leilungsbahnen, pp. 704, 709 ; see also " Hist. 

 Beitr." v. p. 76) in coniferous tracheids. The best evidence for 

 this seems to be the fact mentioned {ibid. p. 79), that the part 

 of a single tracheid in front of an air bubble gets red with 

 absorbed eosin, though the neighbouring tracheids are colour- 

 less. This clearly suggests the creeping round the bubble 

 which Strasburger believes in. Schwendener {Zttr. Kritik, 

 &c., p. 921) has been unable to confirm Strasburger's micro- 

 scopic observations, and, moreover, denies the physical possi- 

 bility of the phenomena. I am unable to judge of the validity of 

 Schwendener's theoretic objections, and must leave this point. 

 It is a question of great importance whether it is possible that, 

 on the breaking of a column of water, a film of water remains 

 surrounding the air bubble, and capable of holding the two 

 columns together. If this is impossible, we must suspend our 

 judgment until we know more of the contents of the tracheals. 



To sum up this part of the subject, we may believe that the 

 tracheals in their youngest condition may contain water in 

 continuous columns, since the cambium cells from which they 

 arise certainly contain fluid. But we know also that this con- 

 dition is not absolutely maintained, since Strasburger has 

 shown that the young wood contains air, though in small quan- 

 tity. We must, therefore, believe either (i) that the transpira- 

 tion current is able to travel past the air bubbles, or (2) that 

 tracheals partly filled with air may again become continuous 

 water-ways by solution of the air. If we adopt the first alterna- 

 tive, we must believe that the film of water between the bubble 

 and the wall of the vessel is able to bear such a tensile stress 

 that it can serve to link the column above with the column below 

 the bubble. But this is analogous to trusting a rope so nearly 

 cut through that only a few threads remain intact. With regard 

 to the second alternative, we have, at least, indications from 

 Strasburger's work that a tracheal partly filled with air does not 



L ^'^'l^*"S^^'^linen, p. 683 et. seq. ; Russow in 1882 (^Bot. Centr.) vol. xiii. 

 1883) observed similar facts in the distribution of water and air. 

 .- Vcrhand. Nat7trkist. med. Vereins Heidelberg;, 1895,?. 'S- 



NO. TJ48, VOL. 56] 



necessarily remain permanently functionless (see Leilungsbahnen, 

 p. 692). 



The isolation of the tracheals. — There are a number of points 

 connected with the structure and properties of wood which 

 ought to be considered in relation to the modern theories. 

 Want of space forbids my doing more than referring to two of 

 them. 



The resistance which the wetted cell-wall offers to the passage 

 of undissolved air is a point on which many writers have laid 

 stress. It is clear that on any theory of the movement of water 

 in the tracheals, it is essential that air should not filter into the 

 water-way. This necessity is not, however, stronger in the case 

 of the modern theories we are considering. The pressure tend- 

 ing to fill the tracheals with air from outside cannot be greater 

 than atmospheric pressure, and since the wetted cell- walls of 

 gymnospermous wood can resist the passage of air under a pressure 

 of about an atmosphere, 1 we need not fear criticism of the theory 

 on this ground. The above remarks seem, however, to be 

 needed in face of the frequently recurring statement that wet 

 wood membranes are impermeable to free air. Schwendener 

 has some good remarks on this head {Ztir Kritik, p. 943). 



Strasburger has called attention to the important subject of 

 the localisation or isolation of vessels, or of certain lines of 

 tracheids. When this is possible we may have one set of 

 tracheals containing continuous water columns, while neigh- 

 bouring ones contain air at negative pressure (see Histolog. Beit- 

 rage, V. p. 87). This is especially important in connection 

 with the Dixon-Joly- Askenasy theory, since, if there were no 

 such isolation, a functioning tracheal containing a continuous 

 column of water would give up its water to one which was not 

 functioning. In other words, the inactive tracheals would, by 

 negative pressure, suck water from the active ones. In the 

 coniferous trees, the young wood is cut off by the absence of 

 pits in the tangential walls '■^ from free communication with the 

 older wood, where air is more frequent. 



In the same way the valve-like closure of the pits by the 

 aspiration of the pit membrane, comes to be a subject of much 

 importance. 



At present I merely wish to show by a couple of examples the 

 necessity of a complete study of the minute structure of wood in 

 relation to the modern theories. It is, at least, a hopeful fact for 

 Messrs. Dixon, Joly, and Askenasy that we cannot point to 

 anything in the anatomy of wood which is absolutely inconsistent 

 with their views. Finally, with regard to the question at large, 

 whether we are friends or opponents of Messrs. Dixon, Joly, and 

 Askenasy's theory, the broad facts remain that water has the 

 power of resisting tensile strength, and that this fact must 

 henceforth be a factor in the problem. There are difficulties in 

 the way of our author's theory, but it is especially deserving of 

 notice that many of these difficulties are equally serious in the 

 case of any theory which excludes the help of the living elements 

 of the wood, and assumes a flow of water in the tracheals. The 

 authors have not only suggested a vera causa, but have done 

 so without multiplying difficulties. There is, therefore, a dis- 

 tinct balance in their favour. 



Huxley, quoting from Goethe, makes use of the expression 

 thdtige Skepsis. It is a frame of mind highly appropriate to us 

 in the present juncture, if we interpret it to mean a state of 

 doubt whose fruit is activity, and if we translate activity by 

 experiment. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Mrs. Randall, of Massachusetts, has bequeathed to Rad- 

 cliff College (the Harvard annex for women) 20,000 dollars ; 

 an equal amount to the Prospect Union, and 70,000 dollars to 

 the Foxcroft Club of Harvard. 



The Council of University College, Liverpool, on the re- 

 commendation of the Senate and Medical Faculty, have con- 

 verted the Medical Lectureship of Hygiene into a Professorship 

 of Public Health, and have appointed Dr. E. W, Hope, 

 Medical Officer of Health to the Corporation of Liverpool, to 

 the chair. 



1 Leilungsbahnen, p. 722. Nageli and Schwendener, Das Mikroskop, 

 2nd edit. p. 367, give 225 cm. of mercury. 



2 Strasburger discusses, in this connection, the existence of tangential 

 pits in the autumnal wopd (see Leiiuitgsb.ahnen, p. 713). 



