Physiological Functions of Plant Tissues. 267 



Thirdly, The ordinary dicotyledonous Bystem, in which 

 the spongy parenchyma connects the assimilating cells with 

 the parenchymatous sheaths round the bundles which 

 conduct the assimilation products away. (This is beautifully 

 seen in Ficus elastica). 



(c) Conduction System Pi'oper. — There are three chief 

 streams always flowing through the plant. The water with 

 salts in solution flowing from the roots utilised as cell sap ; 

 and two distinct streams from the leaves directed to ail 

 parts, one of them conveying the carbohydrates and 

 asparagin, whilst the other takes the various albuminoids. 



He considers that the water from the roots is carried by 

 the vessels and tracheids of the wood, and for this there is 

 the following evidence. Hohnel showed experimentally 

 that tljere is no communication between the vessels and the 

 intercellular spaces, stomata, or lenticels ; Bohm and 

 Hohnel, and especially Volkner, state that the vessels always 

 contain water at night only, and during the day air at a low 

 pressure. Elfving, by means of eosin solution, showed the 

 taking up of fluids by the tracheids ; and he also injected 

 melted cocoa butter into the branches of several plants, and 

 found that after cooling it was impossible to inject water by 

 pressure. He points out also the uninterrupted continuity 

 of these vessels from the roots to the leaves, and their much 

 smaller development in aquatic plants. The various thick- 

 enings would on this theory be a means of strengthening the 

 tubes, without interfering with the percolation of water out 

 of them. The dotted ducts he considers as mechanical 

 valves which prevent the backflow of the water into the 

 vessels when too much has been drawn from them by the 

 active transpiration during the day. 



On the other hand, the carbohydrates and amides are, 

 according to him, conveyed by the parenchymatous sheaths 

 of the smaller vascular bundles to the parenchyma of the 

 nerves, and thence by the woody and medullary parenchyma 

 through the plant. 



The albuminoid substances, again, are conducted through 

 the sieve tubes andcambiform cells. These latter contain a 

 parietal layer of finely granular protoplasmic matter, a band 

 of protoplasmic matter, and in the centre a clear fluid with an 

 alkaloid reaction. The strong development of soft bast in 



