26o 



SCIENCE PROGRESS 



the leaf by way of the parenchymatous sheath surrounding 

 the vascular bundles of the veins (figs, i, 11-13), the vascular 

 bundles themselves being unessential for this purpose. Schim- 

 per's work was generally regarded as imparting an air of finality 

 to this view, until in 1897 Czapek published the results of some 

 experiments devised to investigate the sugar-conducting function 

 of the sieve-tubes. He stated that although a certain amount of 

 diffusion goes on through such a tissue as Schimper's bundle 

 sheath, yet for comparatively rapid translocation over long 

 distances the sieve-tube part of the bundle furnishes the sole 



z. 



Fig. I. — Transverse section of leaf of Fiats elastica. 



A, palisade cells ; B, collecting cell ; C, cell of spongy parenchyma ; D, parenchymatous bundle 



sheath surrounding small bundle. 



(After Schubert.) 



Fig. 2. — Diagrammatic longitudinal and radial section of a Dicotyledonous stem 

 to show relations of tissues. 



Ep, epidermis ; Co, cortex ; En, endodermis ; F, fibres ; p, parenchyma of the bundle ; 

 S, sieve-tubes and companion-cells ; C, cambium ; V, vessels ; P, pith. 



path for the assimilates as a whole. This conception of the sieve- 

 tubes as the elements concerned whenever elaborated materials 

 travel more rapidly than is possible by simple diff'usion from 

 cell to cell, was criticised by Haberlandt, the chief living exponent 

 of the theory of separate paths. He devoted a page of the third 

 edition of his well-known Physiologische Pflanzenanatornie (1904) 

 to a criticism of some of Czapek's work. In this he showed how 

 some of the results might be made to support his own views, 

 laid stress upon the anatomical features of the assimilatory and 

 conducting tissues which he himself had so carefully studied, 



