THE FORMATION OF GERM-CELLS 219 



a latent condition, and these are ready to become active as soon 

 as a suitable stimulus is brought to bear upon them. This 

 proof he considers to be incompatible with the assumption of 

 the existence and continuity of a germ-plasm. 



The development of galls is undoubtedly a highly interesting 

 problem, which, in my opinion, has not yet been fully explained, 

 in spite of the numerous and excellent researches on the subject 

 which have been made within the last ten years. Amongst 

 these, the contributions of Adler * and Beyerinck f in particular 

 have materially helped to throw light on the problem. 



The most important point in the consideration of this question 

 is the fact that galls are not by any means exclusively composed 

 of those kinds of cells which occur in the organs of the plants 

 upon which they arise, but may also contain cells of other kinds. 

 ' Cells which are usually only developed in the bark of a plant 

 may also frequently be found in the galls produced by those 

 CynipidcE and Dipt era which infest leaves.' It is therefore cer- 

 tain that the power of producing forms of cells which do not 

 usually occur in the leaf, for instance, ' is not confined to those 

 organs in which they are developed normally,' but is present 

 also in certain cells of the leaf, and even indeed, de Vries thinks, 

 • in all other parts of the plant.' 



This is not surprising if we look upon the formation of the gall 

 as due to an adaptation of the plant to its parasites, such as we 

 may assume to have occurred with regard to the peculiar 

 arrangements exhibited by certain tropical plants for the pro- 

 tection of ants, which in their turn again protect the plant. 

 Reciprocal adaptation has taken place in this case ; the animal 

 has become adapted to the plant, and the plant to the animal, 

 because a joint existence is advantageous to both of them. In 

 the case of the galls of the Cynipidce and TenthrediiiidcE, the 

 advantage which might result to the plant from the presence of 

 the parasite is not apparent, and we may therefore be inclined 



* Adler, ' Beitrage zur Naturgeschichte der Cynipiden," Deutsche 

 entomolog. Zeitschr. xxi., 1877, p. 209 ; and ' Uber den Generations- 

 wechsel der Eichengallenwespen,' Zeitschr. f. wiss. Zool., Bd. xxxv., 1880, 

 p. 151. 



t M. N. Beyerinck, ' Beobachtungen iiber die ersten Entwicklungs- 

 phasen einiger Cyidipidengallen,' Akademie d. Wiss. zu Amsterdam, 1882; 

 ' Die Galle von Cecidomyia poa;,' Bot. Zeitung, 1885 ; ' Uber das Cecidium 

 von Xematiis capre?e,' Bot. Zeitung, 1888, Xo. i. 



