268 SCIENCE PROGRESS 



a non-climbing species of the same genus. For example, in 

 Aristolochia Sipho (climber) and A. clematitis (non-climber), the 

 diameters of the largest vessels were found to be 200 /i and 70 fi 

 respectively. But he found further that this rule could not be 

 applied to sieve-tubes, for in Lonicera and in Clematis " the 

 phloem is identical whatever the habits of the species." More- 

 over certain climbing plants were found to have very narrow 

 sieve-tubes, while those of the succulent Stapelia, of the bogbean 

 {Me)iya)ithcs trifoliata) and those of the rhizomes of Mono- 

 cotyledons showed comparatively wide lumina. Unfortunately 

 Herail concluded from these observations that " the structure 

 and particular position of the phloem elements have no physio- 

 logical relations," a conclusion which Lecomte considered to be 

 erroneous, since the habit of growth is not necessarily the 

 dominating factor in the whole physiology of the plant. As a 

 result of further investigations Lecomte concluded that the 

 number and area of the leaves, the amount of phloem contained 

 in the stem, the place of storage or utilisation of assimilates, the 

 size and the affinities of the plant, all have to be taken into 

 account when considering the diameter of the sieve-tubes. 

 Generally, when there is a great production of assimilates, as 

 in plants bearing many or large fruits, the sieve-tubes are well 

 developed ; when the assimilating and transpiring surfaces 

 are large the diameters of the sieve-tubes and vessels are also 

 large, especially if the stems are slender. 



Kienitz-Gerloff, in 1902, remarked on the great amount of 

 food material which had to be conducted in a short time through 

 the tissues to form a Gourd and correlated this with the structure 

 of the sieve-tubes, apparently so favourable to rapid conduction 

 through the relatively slender stem. 



Similarly, in the peduncle of the fruit of Artocarpus incisa 

 (the bread fruit), the sieve-tubes are extremely large and numer- 

 ous. This fruit is of considerable size and forms rapidly, so that 

 here again there is a rush of material through the peduncle. An 

 examination of the structure of this showed that the general 

 ground tissue is composed of cells which are often flattened 

 rather than elongated parallel to the axis. Their walls are not 

 thin but are freely pitted, and it would seem that the chief 

 function of this parenchymatous tissue is to provide the necessary 

 mechanical support for the heavy fruit. Apart from the latici- 

 ferous tubes present, it certainly looks as if the only elements 



