STRUCTURE. 69 



The palisade layer of waterlily leaves is very deep, and composed of 

 parallel strings of cylindrical cells. Between these are long, narrow air- 

 chambers, opening through the stomata above, and connecting with the 

 lacunae of the spongy mesophyll below. In N. tetragona the palisade layer 

 is about 0.09 mm. deep, in N. rubra (Fig. 30) o. i to o. 14 mm. in depth, being 

 in each case one-half the thickness of the whole mesophyll, and consisting 

 of two to three series of long, narrow cells. In N. odorata the palisade 

 occupies a little more than one-third of the thickness of the mesophyll 

 and consists of five to seven series of cells with an aggregate depth of 

 0.14 to 0.15 mm. In N. tubcrosa there are four or five series of cells 

 with a depth of about o. 16 mm., or about one-half of the mesophyll. JV. 

 zanzibariensisY. has five series of cells, making about 0.14 mm., or a little 

 less than half the mesophyll ; finally, N- amazonum has four series of cells, 

 altogether about o. 14 mm. deep, or half the depth of the mesophyll. Where 

 the palisade layer is more than two cells thick, the uppermost ones are 

 shorter than the rest, often but little longer than wide. Ameling (1893, 

 p. 189) gives 0.0166 mm. as the average diameter of a cell of the palisade 

 parenchyma of N. alba, and points out the interesting fact that this is 

 exactly the same in a leaf 230 mm. in diameter and in one only 95 mm. 

 across. In another table (p. 198) a leaf 190 mm. across had the palisade 

 cells 0.019 mm - m diameter, and this is compared with a like measurement 

 of 0.0197 mm. in a leaf of Victoria regia 900 mm. across. The close 

 agreement in size of cells is striking. The lower ends of the strings of 

 palisade cells rest against a very loose, irregular layer of mesophyll cells, 

 the upper members of the spongy parenchyma (Fig. 30). They are large, 

 thin-walled and turgid, and of the most varied forms ; they connect with 

 trabeculae or rods and plates of soft cells which pass vertically from this 

 layer to the lower epidermis. Great air-chambers lying between the 

 trabeculae occupy five-sixths of the space of the spongy layer. Bounding 

 and supporting the lower epidermis in N. odorata, tuberosa, flara, 

 marliacea chromateUa, and zanzibariensis x is a i to 3 seriate layer of 

 smaller, fairly regular, and close-fitting parenchyma cells. This layer may 

 be absent in N. rubra, ainazonnni, and tetragona, the air-spaces of the 

 mesophyll being bounded below only by the epidermis. Immediately over 

 the insertion of the petiole the mesophyll cells are nearly cubical, with 

 walls evenly colloid-thickened, and without intercellular spaces. 



A very striking feature of the mesophyll is the presence of innumera- 

 ble idioblasts. In the spongy layer these are usually of the multipolar or 

 stellate form. They project from every side into the air-spaces, and 



