44 Henry H. Dixon. 



liere. These results do not tell us how the air is distributed, and it 

 is evident that 10 %, of air occurring- in each trachea might effectively 

 destroy the cohesion of the transpiration current, while 50 ^/o placed 

 in half the tracheae would only diminish the maximum transmissibility 

 for a g-iven tension to one half. 



Attempts to estimate the number of the vessels and tracheids 

 which contain air have been made on various occasions ^) ; but 

 unfortunately, as we shall see later, all the methods hitherto adopted 

 are probably open to error. Of these Strasburger's^) results seem 

 to be those obtained with the greatest care. His method was as 

 follows: A branch cut from a tree under water, was kept for half 

 an hour in a dark moist space, in which transpiration was reduced 

 to a minimum. In this way the gas pressures in the branch were 

 brought into equilibrium with the atmosphere. From the middle 

 region of this branch a piece about 10 cm long was then cut under 

 water, and this piece while still submerged was split down the middle. 

 The flat surface thus made was raised out of the water and covered 

 with a viscid fluid, e. g. olive oil, glycerine or a mixture of glj^cerine 

 and gum. It was then ready for microscopic examination. Stras- 

 burger took account only of uninjured tracheids and, as far as 

 possible, of uninjured vessels. Consequently he gives most weight to 

 his observations on the conducting tracts of the Coniferae. 



He examined in midsummer branches of Picea, Pinus, Larix, 

 Abies and Tsuga. All of these showed that very few bubbles were 

 present in the tracheids of the outer year rings and those occurring 

 there were for the most part confined to the tracheids of the late 

 or summer wood. 



An examination of the wood of Pinus, Larix and Abies made in 

 winter gave generally the same result except that the scattered 

 bubbles were found in the tracheids of the spring as well as the 

 summer wood. A frozen branch of Tsuga taken at the same time 

 contained air bubbles in many of the tracheids, even of the outermost 

 year ring after it had been thawed out for examination. 



Strasburger also records observations on a few dicotyledonous 

 trees. Thus branches of Robinia, Wistaria and Quercus in summer 

 showed very few bubbles in the larger vessels of the last year ring, 

 in the next somewhat more, and a still further increased number in 

 the third and so on. Special attention was paid to the air-contents 

 of the vessels of Ficus, Acacia and Salix. In these genera, Stras- 



und des Luftraumes in den Bäumen, und über die Ursache der Wasserbewegung' in 

 transpirirenden Pflanzen. Berlin 1882. 



1) Fr. El f vin g , üeber die Wasserleitung im Holz. Bot. Ztg., 42. 1882. 



*) E. Strasburger, Ueber den Bau und die Verrichtungen der Leitungs- 

 bahnen in den Pflanzen. Jena 1891, pp. 677 et seq. 



