158 



is caused by a destruction of the chloroplasts in the affected 

 cells. In flg. 12 part of a section of a yellow and distinctly 

 thickened leaf-tip has been drawn 400 times magniâed. It 

 may be seen, that the destruction of the chloroplasts has 

 gone furthest in the cells of the spongy parenchyma in which 

 only slight, yellow fragments of the chloroplasts are to be 

 found; such remainders find themselves also in the sub- 

 palissade cells although the destruction is less there than 

 in the spongy parenchyma. Best preserved the chloroplasts 

 remain in the palissade cells, in which one finds besides 

 yellow fragments and chloroplasts that are more or less 

 swollen and discoloured such as hâve yet their normal 

 color and shape at the time the other cells contain only 

 those yellow shapeless rests. 



Since the cells from which they are built up, divide 

 parallely to the surface of the leaf and the so-formed divi- 

 sion-cells stretch themselves again, thèse spécial thickenings, 

 which at first appear as small unevennesses. grow out into 

 the knobs, already mentioned on p. 152. 



How has the epiderm been able to follow the increase 

 of surface, accompanying thèse thickening processes? 



In a normal leaf-tip we find for the dimensions of the 

 epiderm cells 9 to 18 ," height and 9 to 29 /" breadth, 

 while in the epiderm, covering a spécial thickening, amidst 

 cells of normal dimensions others are found which 

 measured : 



Height fi: 9 11 

 Breadth /x: 31 36 



13 

 45.5 



13 

 36 



13 



49 



9 11 



45.5 45.5 



11 



42 



9 9 

 54.6 45.5 



Hence some epiderm cells seem really to broaden ; whe- 

 ther this is only an extension or active growth, I dare 

 not décide. 



Besides, the epiderm soon gives way and is rent. Like 

 the part of the epiderm which gives way to the pressure, 

 some cells of the tissue underneath die off, the cell-walls 



