278 SCIENCE PROGRESS 



By observing the changes in amount and size of the grains 

 of starch in the sheath at various stages in the growth of the 

 plant, he was led to conclude that the main function of this 

 starch was to form a reserve for the purpose of thickening the 

 fibres usually present outside the phloem (fig. 2). When these 

 were well developed he found that the starch content of the 

 sheath had diminished and this in proportion to the amount of 

 thickening. It would appear that Schimper accepted this view, 

 but Czapek remarks that the starch sheath must have some other 

 function as well, since "... adequate bast fibres can be 

 developed without the presence of a starch sheath." He agrees, 

 however, that it is most probably not concerned in the longi- 

 tudinal conduction of sugars. 



Accepting the above view that the assimilates enter the sieve- 

 tubes of the finest veins directly from the single-layered sheaths, 

 it will be evident that there is no demand for any translocatory 

 tissue outside the bundles of the stronger veins. Their nerve 

 parenchyma, though continuous with the small sheaths, has an 

 obvious mechanical function, a function which the smaller 

 sheaths are not required to perform. Thus, strictly speaking, 

 the two tissues are neither morphologically nor physiologically 

 homologous. 



Continuity of Leaf and Stem Tissues. — There are a number of 

 other anatomical facts which render it highly unlikely that 

 translocation (as distinct from slow diffusion) goes on through 

 the tissue outside the bundles of veins and petioles, at least in 

 some plants. Strasburger pointed out that in the Palms the 

 connection of the tissue of the leaflets with that of the main 

 leaf stalk is such that all other translocation than through the 

 tissue of the central cylinder is excluded. In the Conifers the 

 leaves are so much constricted at their bases that only the tissue 

 of the vascular bundles passes into the stem. Furthermore, in 

 many Dicotyledons a layer of cork, extending to the vascular 

 bundles, forms across the petiole early in the year, in preparation 

 for the fall of the leaf. Such a transverse layer forms in 

 ^sculus Hippocastannm early in July, whilst the leaf does not 

 fall till the autumn. Also in Veronica Hectori and other species, 

 according to information given me by Mr. R. S. Adamson, a 

 layer of cork forms at the base of the leaf during the first year, 

 while the leaf itself functions for three years or so. 



Cases like these make it very improbable that in such plants 



