200 SIGNIFICANCE OF MALFORMATIONS IN ORGANOGRAPHY 



sclerenchyma and vascular bundles are produced. There is a lid to the gall, 

 which, produced without any special aid of the insect, enables it at a later 

 period to leave the gall-chamber which served for its protection. 



In another gall protection is provided by the development of roots 

 in consequence of the stimulus of the larva, and these grow over the 

 larval chamber, stick to one another and make a dense living mantle. This 

 is what is found in the galls of Cecidomyia Poae upon Poa nemoralis l . 

 The roots arise here at places where under normal conditions they would 

 never appear. The gall-forming stimulus passes in this case from the 

 larva which is firmly fastened to the surface of a still growing internode ; 

 cushion-like swellings are first of all formed right and left of, but at some 

 distance from, the larva, and from these the roots then take their origin, 

 and these roots can be caused to develop further as normal roots by 

 using a gall as a cutting. The behaviour of Nematus Capreae 2 shows 

 however that the gall does not originate in every case through the exercise 

 of a stimulus of the larva. In this case, as Adler found, the facts agree 

 with the old theory of Lacaze-Duthier ; the development of the gall is 

 dependent upon the substance introduced with the egg into the young 

 leaf from the poison-bag ; the formation of the gall is evoked by any 

 wounding effected by the saw of the insect even though no egg is deposited, 

 and the artificial destruction of the egg does not here arrest the gall- 

 formation, the development of the egg only affects the size of the gall. 



In concluding this short reference to recent investigations into the 

 formation of galls I would only emphasize these two points 



1. In general no tissue-elements appear in the anatomical structure of 

 the gall which do not exist elsewhere in the plant under other conditions ; 



2. All the more highly differentiated galls are produced out of juvenile 

 tissues still capable of development which are caused to develop in an 

 abnormal way by the influence of a gall-insect ; the more complex in 

 structure a gall is, the earlier must the influence producing it be exerted 

 upon the plant-tissues. 



III. 



SIGNIFICANCE OF MALFORMATIONS IN THE THEORY 

 OF FORMATION OF ORGANS. 



I have already pointed out some general reflections which arise from 

 the study of malformations, and I will now shortly state to what theoretical 

 considerations they have led. 



We must in the first instance refer to Sachs' idea of ' material and 



1 Beyerinck, in Botan. Zeitung, 1885, p. 305. 



3 Beyerinck, Uber dasCecidium von Nematus Capreaeauf Salixamygdalina, in Botan. Zeitung, 1888. 



