3o6 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



not extend into the central cylinder and the conducting tis- 

 sues, so far as I have seen. Instead, in the layer of cells 

 just outside the endodermis of the root, division takes place 

 in the cell into which the infection thread has penetrated 

 and in the cells adjacent to it. The daughter-cells grow, 

 repeated divisions and growth follow, and there arises a 

 conical mass of cells which are somewhat larger, and which 

 contain more protoplasm, than the adjacent cortical paren- 

 chyma cells. This conical mass is the young tubercle. At 

 first all of its cells are merismatic, but later the divisions 

 become more and more limited to the cells near the rounded 

 apex of the blunt cone. Thus a regular cambium is differ- 

 entiated in the tubercle. This cambium, as shown in fig. 7, 

 lies near the tip of the tubercle, and forms a bowl-shaped 

 or shallow thimble-shaped layer. 



The growing tubercle pushes out the overlying cortical 

 parenchyma and epidermis, forming an increasing swelling 

 on the side of the root. Cortical parenchyma and epider- 

 mis, at least for a time, nearly keep pace with the growth 

 of the tubercle. Thus, although the cortical cells are com- 

 pressed somewhat, the epidermis is not ruptured, and the 

 tubercle does not burst out of the side of the root as a 

 lateral root does. 



The layer of cells which, on infection, gives rise to the 

 tubercle, forms new cells not only centrifugally but also 

 centripetally, so that by these new and growing cells the 

 tubercle is pushed outward, away from the central cylinder. 

 In this way the cylindrical mass of the root itself is kept 

 fairly uniform. An older tubercle appears to be attached 

 to the root. Only by tracing its development can one see 

 that it originates internally. The course of development 

 was only very imperfectly traced by Schneider (1893); 

 hence his bold and erroneous statement that "tubercles 

 seem always to develop exogenously." 



Frank (1890, p. 70) states that the tubercles are new 

 organs formed and well nourished by the plant. He com- 

 pares them with galls which are formed by plants at the 

 points attacked by parasites (insects, worms, etc.), and 



