198 SIGNIFICANCE OF MALFORMATIONS IN ORGANOGRAPHY 



of a three-sided pyramidal apical cell. The first stages of the develop- 

 ment are unfortunately unknown, but they probably arise as growths on 

 juvenile shoots in consequence of the stimulus induced by the larva. We 

 have no ground for assuming that the ancestors of this Selaginella have 

 at any time had shoots with six rows of leaves, or that the ancestors of 

 Pteris quadriaurita developed adventitious shoots upon their leaves l ; 

 we must rather believe that these malformations are new formations 

 whose nature depends on the one hand upon the protoplasm of the plant, 

 and on the other upon the stimulus acting upon it. Lower plants, 

 especially fungi, appear to be more plastic in regard to the production 

 of malformations than are the higher plants in which, as we have seen, 

 only a derangement of the formation of organs occurs. 



What has just been said regarding the malformations caused by 

 insects leads me to cast a glance upon the more recent investigations 

 into the occurrence of galls. These wonderful structures have from an 

 early time attracted attention. We observe in them the formation of 

 a new structure in consequence of a stimulus exerted by an insect, and 

 it serves in an exceedingly effective manner for habitation, nourishment, 

 and protection to the developing insect. Whence proceeds this stimulus? 

 According to Lacaze-Duthier 2 , with whom Darwin and Hofmeister agree, 

 a poison which is introduced by the ovipositor of the insect when it lays its 

 eggs is the cause of the formation of the gall. Of course a susceptibility to 

 the stimulus exercised by the insect must exist in the plant, for we find 

 that the ' virus ' of Cynips Rosae, for example, has no effect upon the oak 

 although this tree produces so many different galls. The theory of Lacaze- 

 Duthier has been however proved untenable in many cases through the 

 famed entomological investigations of Adler. Beyerinck 3 too has enriched 

 our knowledge of the formation of galls, especially from the botanical side. 



It has been shown in the case of Cynips-galls that the stimulus which 

 causes the formation of the gall is not given by the wasp which lays the 

 eggs but by the larva ; and the larva exercises its influence while still 

 completely enclosed in the egg. We can hardly think of this as occurring 

 in any other way than by the excretion from the larva of a soluble 

 substance which penetrates into the tissues of the plant, for the larva is 

 often separated from the tissue developing into a gall ; it may even be 

 divided from this in some cases by dead tissues, and yet the. origin of 

 the gall is not retarded. This shows that we have not to do with the 

 conduction of a stimulus through the living protoplasm of the host-plant. 

 Inasmuch as the influence proceeds from the larva and is a slow one, the 



1 See page 193. 



2 Lacaze-Duthier, Recherches pour servir a 1'histoire des Galles, in Ann. d. Sciences Nat., 1853. 



3 Beyerinck, Beobachtungen iiber die ersten Entwicklungsphasen einiger Cynipiden-Gallen. 

 Amsterdam, 1882. 



