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cells stretch themselves again, these special thickenings, which at 

 first appear as small nnevennesses, grow out into the knobs, already 

 mentioned on p. 3. 



How has the epiderm been able to follow the increase of surface, 

 accompanying these thickening processes? 



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

 cells 9 to 18 ft height and 9 to 29 ft breadth, while in the epiderm, 

 covering a special thickening, amidst cells of normal dimensions 

 others are found which measured : 



Ilti-ht ,« : 9 

 Breadth ,u. : 31 



Hence some epiderm cells seem really to broaden ; whether this 

 is only an extension or active growth, I dare not decide. 



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 turning brown. In this way 

 arise the brown streaks on the surface of the knobs which finally 

 by extension in tangential direction of this suberizing process becomes 

 entirely brown. A special suberizing meristem, a phellogen, is not 

 formed. 



The regular structure of these cell-hills is lost as soon as the dif- 

 ferentiation of a meristem commences. Some cells, assembled in a 

 small group, then enter a new stage of strong growth, which makes 

 them conspicuous in the preparations by a more rounded form amidst 

 the adjoining cubical cells. A number of the surrounding cells are 

 compressed by the pressure which these primordial cells cause by 

 their growth, and die. 



Soon the primordial cells divide into a number of small filial cells 

 with extremely thin walls and dense contents, after which the 

 primordiuni has become meristem. 



For answering the question in what place in a knob the meristem 

 is formed and what is the descent of the initial cells, we have the 

 following data. An otherwise 415 to 450 ft thick leaf-tip had by 

 local swelling to about 840 jj, formed a knob, which by a small 

 depression in the middle was, so to speak, divided into two halves, 

 each of which contained a primordium of a meristem. The surface 

 of the knob was entirely suberized to a fairly considerable depth. 

 In one half the primordiuni lay 220 ft below the top of the knob 

 and its cells in all probability descended from the subpalissade cells, 

 in the other half the primordium lay 180 ft below the surface ami 

 was of the same origin as in the former case. 



