TYLOSES 327 



from which they arise should be regarded as reservoirs of carbohydrate 

 material, a fact recognised by the discoverer of these structures. In 

 the rhizomes of Aristolochia Cleuuditis and A. Serpentaria, for example, 

 the tyloses contain so much starch in winter-time, that the vessels 

 appear to be crammed with this reserve-material for great distances. 



While these various hypotheses concerning the functions of tyloses 

 are doubtless applicable to the special cases selected for mention above, 

 they do not provide an explanation of the fact that these structures so 

 frequently develop in uninjured vessels in herbaceous plants, and 

 also in many woody species without reaching a size sufficient to bring 

 them into contact with one another. Such tyloses obviously cannot act 

 as plugs ; starch is also by no means always present in these structures. 

 The author believes that tyloses of this last-mentioned type take some 

 part in the process of conduction, by increasing the surface of contact 

 between the vessels and the neighbouring parenchymatous cells. In 

 this way they might, for instance, accelerate the development of 

 exudation-pressure in the vessels. They might also conceivably excrete 

 sugar into the cavity, or, on the other hand, withdraw certain con- 

 stituents of the sap, thus acting like haustoria, which they indeed 

 resemble in form. This view with regard to the function of tyloses is 

 supported by the following circumstance, which was first noted by 

 Reess : In vessels which live for several years, the formation of tyloses 

 may continue for a long time, just as if the first-formed tyloses gradu- 

 ally became functionless, and hence had to be replaced by freshly- 

 developed organs of the same kind. It is quite evident that much 

 remains to be done towards a complete elucidation of the physiological 

 anatomy of tyloses. 



B. TRANSLOCATION OF PLASTIC MATERIAL. 



The first step towards internal differentation within the conducting 

 system is taken, when separate channels are developed for the conduction 

 of water and for the transportation of plastic substances. The second 

 stage of specialisation involves a further division of labour, whereby 

 the more diffusible compounds are segregated from the less diffusible 

 substances, each group of bodies being relegated to a separate conduct- 

 ing tissue. The readily diffusible compounds are exemplified by the 

 soluble carbohydrates, and by asparaginic a decomposition product 

 of proteins and other amides. These substances travel in the con- 

 ducting parenchyma. Among the substances which diffuse with difficulty 

 the various proteins are the most important. In the Higher Plants 

 these compounds travel chiefly in the sieve-tubes, but are probably also 

 transported to a certain extent in the phloem-parenchyma (cambiform 

 cells). In Pangium edule the hydrocyanic acid which Treub believes 



