NATURE OF PLANTS 41 



tends to round them out. They are unable to expand laterally 

 owing to the adjacent cells but are free to extend towards the 

 looser cells below, and so they become longer and palisaded. 

 Leaves growing in the deep shade show little indication of palisade 



FIG. 26. FIG. 27. 



FIG. 26. Section of a leaf of Rhododendron. Note the compact palisade 

 tissue which results from intense light. 



FIG. 27. Section of a leaf of skunk cabbage, Spathyema. Note the poorly 

 developed palisade tissue and the loose arrangement of the cells of this plant 

 which lives in moist, shaded places. 



structure. Shade and moisture plants are not obliged to conserve 

 the amount of water received and these two forces, feeble light 

 and moisture, produce a larger and looser arrangement of tissues 

 which is favorable to an interchange of gases (Fig. 27). Par- 

 ticularly is this noticeable in aquatics as in the water lilies and 

 many rushes, etc., where the tissue is loose and spongy and 

 permits a ready circulation of gases from the leaves to the roots 

 and to all parts of the plant body even when submerged. This 

 loose arrangement of tissues also renders water plants very 

 buoyant and consequently they are less liable to injury from 

 the currents of the water. Aquatic plants have little need of 

 strengthening and conducting tissues because they are supported 

 4. 



