166 LECTURE X. 



sequence of the filling-up of the pores of the sieve-plate by a formation of 

 callus in the autumn. In the spring the callus is absorbed and the con- 

 tinuity restored (De Bary, Janczewski). 



From the researches of Sachs and others it appears that 

 the sugar and the amides travel in the parenchymatous tissues, 

 and Sachs observed that these tissues have an acid reaction. 

 But the conduction of these substances is, generally speaking, 

 not direct : it is frequently interrupted by their conversion into 

 an insoluble or at least an indiffusible form at some interme- 

 diate stage in their journey. For instance, there is in the 

 petioles and stems of many of the higher plants a layer (seen 

 in transverse section) of parenchymatous cells near the 

 periphery of the ring of fibrovascular bundles or of each 

 fibrovascular bundle, in which, so long as the plant contains 

 any excess of non-nitrogenous organic substance, starch-grains 

 are to be found. On account of the constant occurrence of 

 starch in its cells, this layer has been termed the starch-layer. 

 But a temporary deposit of starch may take place in other 

 cells as well. Thus Briosi has observed the presence of 

 starch-grains in sieve-tubes, they are present also in laticife- 

 rous tissue, and they may be found throughout the parenchy- 

 matous tissue. Again, drops of oil are frequently to be 

 found, especially in the laticiferous tissue, and even in the 

 chlorophyll-corpuscles themselves ; these too may be re- 

 garded as transitory reserve-materials. Finally, the sieve- 

 tubes and the laticiferous tissue contain proteids, for the most 

 part unorganised and indiffusible, for, in the sieve-tubes at 

 least, they are soluble in dilute potash solution but not in 

 water; these proteids are probably to be regarded as tempo- 

 rary reserve-materials. Inasmuch as the sieve-tubes and also 

 the laticiferous cells and vessels are continuous throughout 

 the plant, the contained proteids can travel in them as such 

 from one part of the plant to the other ; but doubtless their 

 distribution to the tissues involves a conversion into a diffusible 

 form, for it has been found that the latex of certain plants is 

 rich in proteolytic ferment. It is worthy of note that Sachs 

 found the contents of the sieve-tubes to have an alkaline 

 reaction. 



