280 KANSAS UNIVERSITY SCIENCE BULLETIN. 



Perhaps the most interesting feature of the leaf is the great 

 degree of variability shown in the distribution of the palisade 

 and spongj' parenchyma tissues. In one continuous cross sec- 

 tion of the leaf may be seen, for a distance, palisade cells on 

 the upper side, with spongy pai'enchyma below; then these 

 conditions may be reversed for a space, the palisade being 

 below and the spongy being above; again, no palisade tissue 

 can be found on either side, or the palisade cells may extend 

 the whole width of the mesophyll. 



As shown in figure 39, a, b, and c, the spongy parenchyma 

 has no fixed region in the leaf where it appears. This tissue 

 averages three cells in depth from epidermis to vascular sys- 

 tem, and is interspersed with many though usually not large 

 intercellular spaces. The cell walls are slightly thicker than 

 those of the palisade cells, and the cells, which are .spheroidal, 

 elongate or ovoid in shape, have an average cavity diameter 

 of .0301 mm. The chloroplasts are relatively large and vary 

 in number from three to eight. 



The palisade cells have the characteristic cylindrical shape, 

 are very thin-walled, and extend from one to five or six cells 

 deep radially. Small but frequent intercellular spaces are 

 found in connection with the palisade tissue. In figure 36 is 

 shown a composite drawing through the leaf tangentially, 

 showing a surface view of the palisade cells. 



The number of chloroplasts in each cell averages twenty- 

 one. They are relatively large in size, projecting far into the 

 cell cavity. The following table, taken from Haberlandt's 

 Physiologische Pflanzenanatomie, and showing the number of 

 chloroplasts found in the mesophyll tissue of a number of 

 plants, will give an idea of the relatively few chloroplasts in 

 the leaf of Gsertneria: 



Plant. Palisade. Spongy. 



Pulmonaria officinalis 85 15 



Ricinns communis 82 18 



Brassica rapa 80 20 



Galeopsis tetrahit 79 21 



Trapxolum viajus 77 23 



Heliantkus annuus 73 27 



Phaseolus multiflorus 69 31 



In the mesophyll tissue, associated with the veins are 

 sclerenchymatous cells, ranging in shape from stone cells to 

 typical bast fibers. 



