334 CONDUCTING SYSTEM 



sieve-pores in the case of climbing plants. It is thus not a mere 

 coincidence, that when we desire to demonstrate the structure of sieve- 

 tubes, or to carry out researches upon these organs, we generally make 

 use of climbers such as Cucurbita Pepo, Lagenaria vulgaris, Vitis vinifera, 

 and Calamus Rotang. According to Westermaier and Ambronn, par- 

 ticularly well-developed sieve-tubes occur also in Humulus Lupulus, 

 Clematis Vitalba, Lonicera Caprifolium, Dioscorea Batatas, Tamus com- 

 munis, in the genus Serjania, in the Passifloraceae, etc. 



The conclusions concerning the functions of leptome-tissue which 

 have been arrived at above, on an anatomical basis, receive strong 

 confirmation from the results of the so-called ringing experiments 1 ' 

 which were first employed for scientific purposes by Hanstein. In the 

 majority of Dicotyledonous stems and branches, the leptome-strands 

 are all extracambial. If, now, an annular zone comprising all 

 the extracambial tissues be removed from a severed twig say of a 

 Willow near its basal end, only a few small adventitious roots sub- 

 sequently arise from the short piece below the decorticated strip. The 

 larger piece, on the other hand, swells out just above the incision, 

 owing to the formation of a mass of callus-tissue, and gives rise to 

 numerous long roots. In this experiment the continuity of the leptome 

 is completely broken, whereas the conducting parenchyma of the wood 

 is left intact ; the result clearly indicates that the protein-compounds 

 required for the production of roots travel exclusively in the leptome- 

 strands, or, in other words, in the sieve-tubes, and perhaps to some 

 extent also in the leptome-parenchyma (cambiform cells). Hanstein 

 further found that ringing a severed branch does not seriously interfere 

 with the access of protein-compounds to the shorter piece, in those 

 Dicotyledons which possess additional leptome-strands in the pith (such 

 as the Piperaceae, Nyctaginaceae, Solanaceae, Cucurbitaceae, etc.). 

 This result is entirely in agreement with the interpretation placed upon 

 the preceding experiment ; for, in the second case, only a portion of the 

 strands that are believed to serve for protein-conduction are severed. 



As regards the nature of the forces that control the movements of 

 protein-compounds in the sieve-tubes, there seems, in the first place, no 

 doubt that the liquid contents of the intact sieve-tubes are under a certain 

 amount of pressure, which causes them to How in the direction of least 

 resistance. When, namely, a petiole or stem of Cucvrbita is cut across, 

 large quantities of slimy protein-material exude from the severed sieve- 

 tubes. With reference to this point, A. Fischer has proved, that the 

 effects of the partial depletion of the sieve- tubes extend backwards 

 from the cut surface of a petiole through one or two internodes at the 

 very least. This observation indicates that the pressure in the sieve- 

 tubes is sufficient to overcome the resistance opposed by a very con- 



