76 TRANSACTIONS AND PIIOCEEDINGS OF THE [Sess. lxii. 



division, and how the dividing cells are arranging them- 

 selves in rows perpendicular to the point of attack in front 

 of, and circumscribing, the infected cells, and, as it were, 

 setting up a barrier between them and the endodermis and 

 the vascular bundles farther in, which give access to the 

 circulating system of the plant. I seem to see in this 

 arrangement an effort of the plant to oppose the advance of 

 the intruder, and prevent, if possible, his gaining access to 

 the vascular tissue within. The way in which one interprets 

 such things is liable to take its form from the preconceived 

 notion he has entertained regarding it. Frank starts with 

 the notion that this is a case of symbiosis, and he imagines 

 the plant as fishing in- the soil for the bacterium, preparing 

 a door for it, and entertaining it within its root as a wel- 

 come guest. 



The manner in which the cells of the root increase and 

 stand in between the infected cells and the vascular centre, 

 pushing them out farther and farther until an excrescence 

 in the form of a warty growth is made, calls to mind what 

 takes place in animal bodies when invaded by parasites 

 such as tubercle. The healthy tissue surrounding the 

 intruder raises • up a wall of defence, and endeavours to 

 encapsule it, and so prevents it spreading. The bacteroids 

 may, perhaps, not inaptly be compared with the phagocytes, 

 which Metchnikof describes as mustering in force around 

 the seat of an invasion, and not only surrounding, but 

 incorporating the invading crowd of bacteria. 



Either view of the matter is not inconsistent with the 

 further development of the nodule, which increases often to 

 a great size, and is usually connected with the root by a 

 narrow neck. Also there is, if not always, at least usually, 

 established a connection between the nodule and the 

 vascular tissue, and the organisms within the nodule 

 increase so as eventually almost to fill it. These organisms, 

 be they bacteria or bacteroids, or both, are bodies rich in 

 albumen, which is a highly nitrogenous compound. 



The source of this albumen is a very vexed question. 

 There are those who hold that the bacteria within the 

 nodule get their nitrogen from the elementary nitrogen 

 contained in the ground air. I l)e]ieve that is the view 

 which is generally entertained. It must, however, seem a 



