r-oo 



NOTES ,.w, 



identity of the companion-cells (and the elements which take their place among 

 Gymnosperms and Pteridophytes) has considerably restricted tin- connotation of 

 the term " cambiform " ; nevertheless typical cambiform tissue can always be readily 

 distinguished from typical phloem-parenchyma (conducting parenchyma). Stras- 

 burger attaches too much importance to the occurrence of transitional stages between 

 conducting parenchyma and cambiform tissue. As a matter of fact, his whole con- 

 ception of vascular structure is based upon the study of secondary tissues, where 

 ordinary leptome-parenchyma (conducting parenchyma) bulks much more largely 

 than cambiform tissue. 



[Most British botanists follow Strasburger's example in this matter, recognising 

 only three categories of leptome-elenients, viz. sieve-tubes, companion-cells and 

 leptome-parenchyma (phloem-parenchyma). ] 



167. Sieve-tubes were first described by Th. Hartig in 1857. Cf. Nageli : Sitzb. 

 Munchen, 1861. Hanstein : Die Milchsaftgefasse, etc., Berlin, 1864. De Bary : 

 Comp. Anat. pp. 172 sqq. Wilhelm : Beitr. z. Kenntn. d. Siebrohrenapparates 

 dikot, Pflanzen, Leipzig, 1880. Janczewski : Mem. Soc. Cherb. 23, 1891, p. 350. 

 Russow : Sitzb. Dorpat, 1882, pp. 257 sqq. Strasburger : I.e. [157]. p. 286. Id. 

 Bau u. Wachstum d. Zellbaute, pp. 57 sqq. Id. P.J. 36, 1901. Id. B.Z. 1901. A. 

 Fischer : Unters. iib. d. Siebrohrensystern d. Cucurbitaceen, Berlin, 1884. Id. 

 Ber. 3, 1885. Id. Ber. sachs. Akad. 1886. Zacharias : 1884. Lecomte : Ann. 

 Sci. Nat,, ser. VII., 10. Poirault : Ann. Sci. Nat., ser. VII., 18, 1893. A. W. Hill : 

 Phil. Trans. 194 B, 1901. Id. A.B. 22, 1908. 



168. Frank (Lehrbuch, 1), and, later, Blass (P.J. 22) have suggested that the 

 function of sieve-tubes is the storage, not the transportation, of protein materials. 

 This theory was rejected in the second [German] edition of this work (p. 341). 



169. As stated hi the text, Czapek (Sitzb. Wien, 106, 1897) has endeavoured to 

 show that the leptome- strands and especially the sieve-tubes are responsible for 

 the conduction of carbohydrates as well as of nitrogenous plastic compounds. 

 Czapek asserts that conducting parenchyma at any rate in petioles and stems 

 is incapable of transporting any appreciable quantity of carbohydrate material. 

 His strongest argument is based upon certain extirpation experiments, in which the 

 continuity of the tissues was interrupted in one-half of the cross-section of the 

 petioles of various plants (by removing a half-disc at a convenient point). The 

 result of this treatment was an entire cessation of the removal of carbohydrate 

 materials from the affected halves of the experimental leaves. Now, the only tissues 

 completely interrupted by the aforesaid operation were the vascular strands ; for the 

 conducting parenchyma (parenchymatous ground-tissue) in the affected half of the 

 petiole remains in lateral continuity with the corresponding tissue on the other 

 side. Czapek infers that the paths along which carbohydrates travel must run 

 straight up and down the petiole, and further concludes that the leptome-strands 

 are the only conceivable conducting channels in the present case. The first of 

 Czapek's conclusions is undoubtedly sound ; but the second is based upon the assump- 

 tion that translocation in the conducting parenchyma takes place with equal readi- 

 ness in the transverse and in the longitudinal direction. Czapek does, in fact, assume 

 that this is the case. But the author himself holds the opposite opinion. Quite 

 apart from the elongated form of the conducting parenchyma cells, a feature which 

 seems clearly to indicate the direction of prevalent translocation it is highly probable 

 that the plasmatic membranes lining the transverse septa are more pervious to 

 plastic materials than those which clothe the longitudinal walls. In the author's 

 opinion, indeed, Czapek's extirpation experiments provide an interesting demonstra- 

 tion of the fart, that, in conducting parenchyma, translocation takes place mos1 



