Comparative Physiology. 571 



sap.* In extracting for examining also, the wounds may cause 

 them to be filled. A great assistant in the ascent, he thought, Avas 

 the alternate contraction and dilatation of the so-called medullary 

 rays, which he proved by experiments were formed from the 

 bark, when all connexion with the medulla was cut off by 

 hardening the alburnum. The office of the true medulla he 

 conceived to be to assist in nourishing the young leaves on the 

 annual shoot. The action of the rays, which he called silver 

 grain, he thought greatly assisted by heat : when the top of a 

 vine inside was excited, the roots outside were found to empty 

 a bottle of water; while other roots with the stem outside, 

 and not excited, did not take up any water in a bottle attached 

 to them. In the Library of Useful Knowledge it is said : " It 

 is not to be doubted the sap ascends by the woody tissue, but 

 whether through the tubes of woody fibre, or by the intercel- 

 lular passages, has not been, and probably cannot be, decided. 

 The dotted ducts and tubes appear to have fluid when the sap 

 is most rapidly flowing in spring, but it is equally certain that 

 the dotted ducts, at least, are empty afterwards. Are we to con- 

 clude that they perform one office in spring, and another after- 

 wards ; or are the appearances of being filled with fluid in the 

 spring owing to the overflow of sap into them when cut 

 through ? There is no satisfactory answer as to this yet on 

 record." The endosmose power of absorption, assisted by eva- 

 poration, and descent of the thickened sap from the leaves, the 

 editor thinks the cause of ascent. Some who experimented 

 with coloured solutions found, if the water contained air, the 

 vessels contained no colour; if the water were boiled, the 

 colour entered the vessels, because there was no air to fill them 

 in the water. 



The subject appears beset with difficulties. The course of 

 the ascent is by all allowed to be most rapid and powerful 

 along the bundles of tubes and ducts in the centre. DeCandolle 

 describes the woody fibre as intermixed and surrounded with 

 ducts ; these bundles of fibro-vascular tissue are likely to 

 assist the ascent by capillary attraction, as the threads of the 

 wick of a candle or lamp feed the flame. Nature is not con- 

 fined to one power in its movements ; there may be many as- 

 sistants in the ascent. The ascent of fluids around cells can 

 never be so direct or forcible as along longitudinal tubes lying 

 in one direction. The first appearance of the rise of sap is 

 always seen in the centre, in young spongioles and annual 

 shoots ; and the principal direction, and greatest force of the 



* The many experiments of Mr. Knight are so carefully executed, and so 

 simply and lucidly explained in his Physiological Papers lately published, that 

 they greatly assist in obtaining a practical knowledge of Vegetable Physiology. 



