324 



W. MAGNUS (1903) was unable to confirm this. In the majority of cases the for- 

 mation of the gall is entirely dependent on the development of the larva. It is 

 quite unknown, however, how the differentiation of the tissues of the gall takes 

 place, whether, for example, the formation of the nutritive layer depends on 

 substances other than those which form the sclerotic layer. 



In general, complicated galls, like those we have been considering, arise 

 from embryonic tissues ; these are at least more readily transformed than full- 

 grown ones. We must leave undetermined the question whether, as BEIJERINCK 

 holds, the gall actually arises from the phloem or whether the cambium takes 

 part in the process. It is important to note that the gall-formation arises 

 often from quite uninjured cells, so that the stimulating substance is obviously 

 diffusible. In favour of this view is the fact that galls are in general concentric 

 to the larva and that the action of the stimulus appears to cease at a certain 

 distance from the centre. HOFMEISTER (1868) also held that the cause of the 

 gall was a fluid excreted by the insect. 



The gall-insect thus provides definite chemical substances which act as 

 s timuli, and the plant responds by forming a gall . These chemical stimuli produce 

 different galls in different plants ; thus the gall produced by Cecidomyia 

 artemisiae on Artemisia campestris differs from that on A. scoparia. From this 



.7* 



Fig- 97- Development of the gall of Dryophantafolii Transverse section through the midrib of the oak. 

 After BEIJERINCK (1882). J, formation of the gall swelling ( W) from the phloem. //, bursting of the cortex. 

 ffl t young gall, almost complete ; gg^ boundaries of the assimilatory tissue of the leaf; sf, sclerenchyma ; 

 ph, phloem ; cf, vascular bundles arising from the midrib ; W, gall swelling ; Kn, canal ; L.K, body of the larva; 

 Eh t egg cavity ; Ik, larval chamber ; ttf, nutrient layer ; ss, sclerotic layer. 



it follows that not only the insect but the plant also plays an important role 

 in gall-formation. It is all the more remarkable that the plant itself makes 

 no use of the galls. 



On the other hand the use of the gall-structure to the animal is obvious 

 The differentiation of tissues seen in Dryophanta-ga.\\s occur also in others; 

 tissues, especially nutritive ones, serving mechanically or chemically as a 

 protection to the insect, are common. Frequently cell-forms appear which 

 are entirely wanting not only in the normal plant but in any of its near 

 relatives. We also find other adaptations whose functions in relation to the 

 parasite are even more obvious, such, for example, as the formation of lids to the 

 galls of Cecidoses eremita (KERNER, 1891, II, 526, Fig. 5). The inhabitants of the 

 galls make use of their hosts to the fullest extent without giving any equivalent 

 in return. The plant, too, makes no attempt to get rid of its parasite, it renders 

 up willingly all the material wanted by it, it even builds it a house, and, in short, 

 treats it as though it were one of its own organs. We may, therefore, conclude 

 that the plant cannot differentiate between the materials formed by the larvae 

 and by its own members, and that, in the normal course of development also, 

 the material influences of one part on another must be of great moment. 



Relationships of another but equally interesting type are to be met 

 with, finally, in lichens, which we may cite as an example of symbiosis. 

 As has already been mentioned (p. 243), each of the organisms which com- 

 bine to form the association obtains some advantage from the union, but 



