118 Theory of the Germ-Plasm 



we study the cell-pedigrees of the plant kingdam, the more 

 we become convinced that there is no qualitative distinc- 

 tion in nature between the cells of the germ-track and the 

 somatic cells. 



12. A Decision Reached Through the Study of Galls 



In the foregoing paragraphs we have repeatedly em- 

 phasized how, on the whole, it is impossible to decide the 

 pending question experimentally. The phenomena of re- 

 production by excised parts of plants make manifest the 

 existence of secondary germ-tracks hitherto unknown; 

 but they do not teach us anything about the nature of the 

 remaining somatic tracks. 



An experiment which we cannot carry through is made 

 by the gall-forming parasites in such a great variety of 

 ways that a glance at their products may be made at this 

 point. The thorough and detailed examinations by Bey- 

 erinck have so far enriched our knowledge in this field, 

 that the whole history of development, as well as the an- 

 atomical structure in the grown condition, is clearly laid 

 before us in the case of all the more important forms of 

 galls. ^^ Two laws, especially important for our purpose, 

 have resulted from these studies. First of all, the galls, 

 even at their highest differentiation, are built up of only 

 such anatomical elements as are otherwise found in the 

 plant bearing them. Only the peculiar layer of stone cells 

 of some Cynipid-galls, which later change into a thin- 

 walled nutritive tissue, forms a hitherto unexplained, but 



30Beyerinck, M. W. Beobachtungen iiber die ersten Entwick- 

 elungsphasen einiger Cynipidengallen. Veroffentlicht Kais. Akad. 

 Wiss. Amsterdam. 1882. The same, Die Galle von Cecidomia Poae. 

 Bot. Zeit. 43: 305, 321. 1885, and Ueber das Cecidium von Nematus 

 capreae. Bot. Zeit. 46: 1. 1888. 



