( 173 ) 



Tn a leaf-tip which macroscopically is distinguished, besides by 

 the yellow colour, by a distinct thickening, the intercellular cavities 

 of the spongy parenchyma are found 1o have entirely disappeared, 

 excepting a small corner here and there. The mutual pressure which 

 the cells consequently begin to exert on each other, causes them to 

 assume a more polygonal shape. The cells, bordering on the sub- 

 palissade cells and often also the rows, turned towards the lower 

 epiderm, show a tendency to stretch themselves in a direction per- 

 pendicular to the surface of the leaf. In many spongy parenchyma 

 cells partition walls have formed. 



The part, played by the subpalissade cells in the process of thick- 

 ening, is generally a very important one. So I found in a leaf-tip 

 in a place, where it was 332 ;< thick (the section of a normal tip 

 is on the average 170 ji)> ceu rows, formed of subpalissade cells, 

 partitioned by two or three walls and measuring 90, 110, 115 and 

 127 n in height. — The palissade rows were in these places not 

 sensibly enlarged. 



That the different tissues also in the same leaf-tip do not every- 

 where play the same part in the process of thickening, appears from 

 the following figures, from measurements in two different places of 

 the same leaf-tip; 



Height of the palissade cells. 

 Height of the subpaliss. cells. 

 Height uf the spongy pareuch. 



.'!('. to 45 // j 



t cells with 

 L' 3 partitions 

 73 to 82/* (mostly partitioned) 



273//. 



27 // non-partitioned. 

 146 a I w i' n numerous 



273 fi. 



partition w; 



The special thickened outgrowths, mentioned on page 3, arc caused 

 by the same processes of proliferation which cause the thickening 

 uf the whole leaf-tip, and which in some places go on with particular 

 activity, while the surrounding region seems to come to rest. 



( hi the upper surface they are formed by locally strong prolife- 

 ration of the subpalissade cells. Sometimes also the underlying cells, 

 originating from the spongy parenchyma contribute to them and then 

 it cannot be ascertained as a rule what part is derived from the 

 subpalissade cells and what from the original spongy parenchyma. 

 In most cases the contribution of the palissade parenchyma to the 

 formation of the special elevations is rather unimportant. 



The special thickenings on the lower surface of the leaf are 

 entirely formed by cells which genetically belong to the spongy 

 parenchyma but for the rest in exactly the same way as those on 

 the upper surface. Since the cells from which they are built up, 

 divide parallelly to the surface of the leaf and the so- formed division- 



