332 



CONDUCTING SYSTEM 



numerous exceedingly minute sieve-pores are present ; for if the cells 

 in question are plasmolysed, the protoplasts hardly ever separate from 

 the transverse walls, with which they are evidently in very close rela- 

 tion. According to Poirault, the sieve-plates of the Pteridophyta are 

 likewise only perforated by very minute pores, similar to those which 

 traverse the limiting membranes of ordinary pits. Poirault was able to 

 place the existence of these fine pores beyond doubt in the case of 

 Angioptcris and Ophioglossum. Among Gymnosperms, also, the sieve- 

 plates though areolated are, as already stated, traversed only by very 



fine pores ; the sieve-fields never become 

 obliterated, as they do in Angiosperms. 



Not unnaturally, the physiological 

 functions of such characteristic organs as 

 the sieve-tubes have long been the sub- 

 ject of speculation. The most widely 

 accepted view is that first put forward 

 by Nageli, which regards the sieve-tubes 

 as organs serving for the conduction of 

 the less diffusible plastic materials, and 

 especially of protein compounds. 168 



More recently, Czapek has endea- 

 voured to prove by experiment that the 

 sieve-tubes are also instrumental in trans- 

 porting carbohydrates, and are thus re- 

 sponsible for the conduction of the greater 

 proportion of the non-nitrogenous plastic 

 materials as well. 160 His results may, 

 however, be interpreted in quite a differ- 

 ent sense, and do not in any case con- 

 trovert the hypothesis maintained by 

 Schimper, the author and others, who believe that the translocation 

 of carbohydrates takes place mainly in the conducting parenchyma. 

 Certainly there seems no cogent reason for abandoning the view of 

 Nageli, according to which the sieve-tubes are principally concerned 

 with the transportation of protein-compounds. 



With regard to the anatomical evidence bearing upon this matter, 

 it may first of all be pointed out that the form and structure of the 

 sieve-tubes, and particularly the perforation of the sieve-plates, are 

 most readily explained by means of Nageli's theory. If it be 

 objected that the pores traversing the sieve-plates are in many cases so 

 narrow, that the pressures which prevail in the sieve-tubes could hardly 

 suffice to force " protoplasm " through them, it may be replied that it is 

 not protoplasm that passes through the sieve-pores, but a solution of 



Fig. 134. 



Large sieve-tube of Lagenaria vulgaris, 

 in L.S., after treatment with alcohol and 

 iodine ; g, sieve-plate ; r, contracted con- 

 tents, x 375. After De Bary. 



