328 THE AMERICAN 3I0NTHLY [Oct., 



epidermis there are oval cells with tliin walls and filled 

 with numerous very minute green spots or "Chlorophyll- 

 grains." These cells are definite in their arrangement, 

 those of the upper side being tall and vertical to the 

 surface and closely laid. They are called "palisade cells." 

 On the lower side, on the other hand, the cells are loosely 

 laid. There are considerable spaces among the cells into 

 which the gases of the air may pass. This compactness 

 of the arrangement of the upper side is the explanation 

 of the deeper color of that side. Besides these large 

 oval cells, there is in the center — in some places, not in 

 j^n — a portion of the vein-system. This runs among the 

 cells and supplies the support needed because of their 

 extreme delicacy. It is among these cells of the leaf, 

 in the spaces between them, that the mysterious chemi- 

 cal process of decomposing carbonic acid gas and build- 

 ing up from its carbon such complex products as sugar 

 is conducted. Such being the case, it is at once apparent 

 that the more of leaf, the more chemical reaction. And 

 with epidermis, with its impenetrable outer wall and 

 supporting tissue prevading the entire organ, almost un- 

 limited extent of leaf is possible. The details of the 

 structure of the supporting tissue cannot be undertaken 

 here, but a word about it will be in place. The cells of 

 which it is composed are apparently spirally-wound 

 tubes. In reality, however, they are not strictly spirally- 

 wound tubes, but tubes whose wall is thickened in a 

 spiral line. This gives strength to the tube and at the 

 same time flexibility, enabling it to bend through a con- 

 siderable distance without breaking short ofl". That the 

 tubes are cells is shown in figure 3, where the ends of 

 two adjoining spiral cells are shown. 



The epidermis is more than a layer of cells with a 

 thickened outer wall. In addition to this, the usual type 

 of cell, there are the various sorts of hairs. Taking 

 plants in general, the number of different kinds of hairs 



