THE FOOD OF PLANTS. 13 



Leaf stalks are generally poor in chlorophyll-containing cells, 

 since they do not for the most part function as organs of assimila- 

 tion, being very differently occupied. Microscopic examination 

 of a transverse section of the leaf stalk of Vitis vinifera teaches 

 that close under the epidermis groups of collenchyma bundles 

 are present, and that between these bundles occurs feebly developed 

 green parenchyma together with cells, of which sometimes many, 

 sometimes only few, contain a red-coloured pigment dissolved in 

 the cell sap. The cortex of the leaf stalk, consisting of the 

 various tissues mentioned, surrounds the ring of vascular bundles 

 and the pith. Examination of the leaf stalks of other plants also, 

 e.g., those of Chenopodium bonus Henricus, shows them to be 

 very poor in green tissue. 



We further prepare a transverse section of leaf stalk of. a 

 Begonia (I examined especially Begonia manicata). Below the 

 epidermis comes a ring of collenchyma, then large-celled ground 

 tissue in which the bundles are not arranged in a circle. The 

 peripheral layers of the ground tissue do indeed contain chloro- 

 phyll, but the chlorophyll grains, although relatively large, are 

 only few in number. 



The green stems of plants, like the leaf-stalks, usually participate 

 only to a very limited extent in the work of assimilation, and 

 hence the bulk of their tissue contains no chlorophyll grains. If, 

 e.g., we prepare a transverse section of a poppy stem, we observe 

 in the centre the pith. Outside this come the vascular bundles, 

 each of which possesses, in addition the wood, a broad zone of 

 soft bast, and a bast fibre region outside this. The medullary 

 rays between "the individual vascular bundles are composed of 

 large cells. In the cortex the presence of a closed cylinder of 

 sclerenchyma is especially characteristic; this is surrounded on the 

 outside by a layer of green tissue feebly developed in proportion 

 to the mass of the stem, and immediately on this chlorophyll 

 parenchyma adjoins the epidermis. We still further examine a 

 transverse section of the stem of Chenopodium bonus Henricus, 

 and- find that under the epidermis collenchyma and green paren- 

 chyma alternate with one another. 3 



1 See Sachs, Lectures on Plant Physiology. 



2 See Heinricher, in Pringsheim's Jahrbiicher, Bd. 15. 



3 Literature on assimilatory tissue : Pick, Beitrage zur Kenntniss des assimili- 

 renden Geicebes armlauUger Pjlanzen, Bonn, 1881 ; G. Haberlandt, Pringsheim's 

 Jahrbiicher, Bd. 13 ; Stahl, Botan. Zeitung, 1880. 



