EXPERIMENTS ON WATER-CONDUCTION 313 



undertaken by Volkens, who worked both with herbaceous plants and 

 with the leaves, petioles and young twigs of various deciduous trees. 

 Volkens's method was to cut a piece 2-3 cm. in length from the stem or 

 twig by means of special double shears, so that the two ends were 

 severed simultaneously. From this piece longitudinal sections were then 

 prepared and examined as soon as possible as dry objects under the 

 microscope ; in this way the presence of water in vessels and tracheides 

 can be readily demonstrated. From observations carried out in the 

 manner described Volkens concludes that " in herbaceous plants the 

 vessels contain nothing but water during the early morning hours, when 

 transpiration is slight or completely in abeyance ; in the course of the 

 day, however, a portion of this water is replaced by air." In the afore- 

 said organs of certain trees (Primus, Platanus) the vessels were found 

 to contain a certain amount of water on the hottest summer days, even 

 at a height of more than 40 feet from the ground. In the case of leaves 

 which had been transpiring actively for some time, the vessels and 

 tracheides of the lamina were usually found to contain short water- 

 columns alternating with air-bubbles, though occasionally the contents 

 consisted solely of air and in a few cases of water alone. The tracheides 

 composing the foliar bundle-ends generally contain nothing but water. 



Both* Schwendener and Strasburger have similarly determined the 

 nature of the tracheal contents by direct inspection, in the case of 

 thicker branches and large trunks. Schwendener obtained cylindrical 

 samples of wood with the aid of a Pressler growth-borer (an instru- 

 ment used by foresters). In order to guard against the entrance of air 

 into the severed vessels and tracheides, the cavity of the instrument was 

 tilled with boiled water or glycerine before insertion ; for a similar 

 reason the sections cut from the cylinder were mounted in boiled water 

 for microscopic examination. Samples were removed in this way during 

 the month of May from trunks of Beech, Oak, Alder, Birch, Willow, 

 and Pine ; in all cases the vessels (or tracheides) were found to contain 

 more or less numerous bubbles of air in addition to water. In the case 

 of the Beech, Schwendener further determined the average lengths of 

 the alternating columns of water and air. According to measurements 

 carried out between 30th May and 23rd June, the average length of the 

 water-columns was found to lie between - 06 and "182 mm., and that of 

 the air-bubbles between - 22 and "392 mm. It should be noted that, in 

 these experiments, the tracheal air was measured at atmospheric 

 pressure. Under normal conditions, however, . as will be further 

 expla : ned below, the air in the vessels is more or less rarefied, so that 

 in the intact elements the air-bubbles are longer than they would 

 appear '",o be from the above measurements. Strasburger likewise 

 concludes that all vessels and tracheides contain air-bubbles as well 



