157 



height. — Tho palissade rows were in thèse places not 

 sensibly enlarged (PI. III, fig. 13). 



Tliat the différent tissues also in thc 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 différent places of the same leaf-tip ; 



Height of the paUssade cells. 

 Hoight of the subpaliss. cells. 

 Height of the spongy parench. 



„ , ^ ] most cells with 

 36 to 45 ^t. ^ _ .... 



j 2 or 3 partitions 



73 to 82 i-i. (mostly partitioned) 

 273 tx. 



Height of tho palissade cells. 27 ^. non-partitioned. 



( with numerous 

 Height of the subpahss. cells. I4t> ^- j partition walls. 



Height of the spongy parench. 273 [x. 



The spécial thickened outgrowths, mentioned on page 152, 

 are caused by the same processes of prolifération which 

 cause the thickening of the whole leaf-tip and which in 

 some places go on with particular activity, while the sur- 

 rounding région seems to come to rest. 



On the upper surface they are formed by locally strong 

 prolifération 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 spécial élévations is rather unimportant. 



The spécial 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. 



As it is shown clearly by a hand-cut préparation ofliving 

 material, the above-mentioned discoloration of the leaf-tips 



