20 



BULLETIN 1038, U. S. DEPARTMENT OF AGEICULTUEE. 



essential change in ratio between spongy and palisade tissue. (PI. VI, 

 figs. G and H; Pis. VIII and IX.) In some cases a slight increase 

 in number of cell divisions also takes place (PI. VI, fig. E). The 

 individual cells appear abnormally healthy, and increase in leaf 

 thickness here is due largely to increase in size of the individual cells 

 (Table I). Along with this increase in cell size there is a reduction 

 in size of intercellular spaces within the spongy parenchyma, and in 

 the severely attacked leaf an almost complete obliteration of air 

 spaces results (PL VI, figs. B to F). 



Table I. 



-Tissue and cell measurements of normal pecan leaves compared with 

 those of leaves hypertrophied icith rosette. 



Variety and leaf tissue. 



Tissue and cell measurements (microns). 



Normal. 



Rosette, about 

 half size. 



Rosette, aborted. 



Rosette, 

 linear. 



rrotscher variety, Thomasville, 



Ga.: 



Palisade cells... dimensions.. 



Spongy tissue. . . .thickness. . 



Spongy tissue where palisade 



tissue is lacking: 



Upper epidermis 



thickness. 



Lower epidernus. . . do 



Van Deman variety, Cairo, Ga.: 



Palieade tissue thickness. . 



Spongy tissue do 



50 to 52 by 5.5 to 6 

 87 to 100 



9.4tol2.6 



10.3 ton 



53 



84 



57 to 60 by 6.5 to 9.5 



76 to 116 



33 to 65 



58 to 67 by 8.5 to 10, 



114 to 116 



61 to 70 



10 to 12.2. 

 10 to 11... 



11.2 to 14. 

 9.4 to 11.. 



69.. 

 125. 



Linear leaves may or may not show a differentiation into palisade 

 and spongy tissue, but where the palisade cells are developed they 

 are usually shortened vertically and the spongy cells of the paren- 

 chyma are more closely packed together than the cells in healthy 

 leaves (PL V, fig. D). The average thickness of these leaves tends 

 to be less than normal and this reduction is due partly to decrease 

 in the number of cell divisions and partly to the shortened vertical 

 axis of the palisade cells. The elongated shape of the leaves, how- 

 ever, is not due to variations in cell shape but rather to a decrease 

 in the number of cell divisionsi in which the central spindles are 

 perpendicular to the midrib. This would tend to keep the cells closer 

 to the main water supply of the leaf. 



Amelung (9) working with plants and Conklin (25) worldng 

 with animals have shown that normal tissue cells of corresponding 

 organs or parts of organs within a species or variety are in general 

 of the same size and that the size of organs is primarily due to 

 differences in the number rather than in the size of cells. Mrs, 

 Tenopyr (Y7), as a result of investigations in several species of 

 plants, found that difference in the shape of leaves of the same 

 plant or related species is not correlated with difference in the shape 

 of their cells. Linear leaves are not composed of longer, narrower 



