167 



it lias been stated that a high température and great 

 humidity of the air are essential factors. 



Experiments enabled me to form an idea about the 

 character of the stimulus exerted by the scale-insects on 

 the tissue of the leaf of Gnetum Gnemon, on which the 

 formation of the intumescences is the reaction. After I 

 had tried artificially to produce intumescences in leaves 

 by mechanically wounding them in ail sorts of ways and 

 treating them with poisons, without obtaining the desired 

 resuit, I arrived at the conclusion that either my method 

 of wounding, compared with that of the scale-insects, was 

 too coarse or that the insect injected some stimulating 

 substance into the leaf. In order to settle thèse points 

 the following experiments were carried out: 1). In the 

 leaf-tips extremely small wounds were made by means of 

 the sterilised, very fine point of an injection syringe. 2). 

 The same was done after the point had flrst been stuck 

 into yellow vesicles, caused by the scale-insects. 3), A 

 number of yellow and thickened leaf-tips were ground in 

 a mortar and a very small portion of the so obtained 

 porridge, mixed wàth some diluted glycerin, injected in 

 several places in leaf-tips. 4). The same opération as in 3 

 was applied after the porridge had first been heated to 100" C. 



The resuit was exactly the same in ail cases. 



After some ten days small, brown specks were visible 

 in the wounded places, which afterwards could still in- 

 crease somewhat in size. A month after the wounding the 

 brown specks had become surrounded by a very thin, 

 more or less transparent, yellow^ margin. The brown specks 

 were formed by the cells which had died in conséquence 

 of the wounding and the walls of which had turned brown. 

 In the yellow margin a complex was found of relatively 

 small cells, leaving no intercellular cavities. Thèse cells 

 had thick walls and their protoplast still contained rem- 

 uants of the chlorophyl grains. The complex was formed 



Recueil des trav. bot. Néerl. Vol. IV. 1907. • M 



