BOT.-VOL. II.] PEIRCE— ROOT-TUBERCLES. 305 



of the cell-walls is well enough known in other cases ; but 

 it is not the host which softens its own walls in order to 

 facilitate the entrance of a foreign organism, but rather the 

 foreign organism which, by enzyms secreted by itself, 

 softens or dissolves the walls of its host which lie across its 

 path of growth.^ 



Having entered the root-hair by softening or dissolving 

 a small portion of the cell-wall, and moving or growing 

 through this, the tubercle bacteria multiply rapidly, forming 

 a thread-like zoogloea from the infection spot along the hair 

 into the epidermal cell of which the hair is a branch (figs. 

 2 and 3). From the epidermis the infecting zoogloea grows 

 fairly straight into the underlying cortical parenchyma. 

 Figure 4, drawn from one of a series of thin microtom sec- 

 tions stained as previously described, indicates the course 

 of the infecting strand (purple). This course is nearly, 

 though not quite, straight toward the central cylinder of the 

 root, for within a series of five or six sections — a distance of 

 20-30 yti — the infection thread was traced from the base of 

 the root-hair (r. h. in fig. 4) to one cell (10) in the layer 

 next to the endodermis of the central cylinder. The cells 

 in this layer are distinguished from the cells of the cortical 

 parenchyma by somewhat larger and denser nuclei. This 

 layer is the one from which the lateral roots arise. The 

 direction of the infection thread — which is solid, and is 

 incorrectly termed infection "tube" — is too regular not 

 to encourage one to suppose that the course of the growing 

 strand of bacteria is determined by attraction exerted by 

 the host-cells upon the bacteria. This then is chemotropic 

 growth of the strand or, if the bacteria are motile in the 

 cells, chemotactic movement of the bacteria. The course 

 of the thread is toward the conducting tissues of the host. 

 This is similar to the growth of the haustoria of Dodder, 

 Cassytha, Vtscum, Phoradendron, and other phanerogamic 

 parasites (Peirce, 1894; Cannon, 1901). The growth does 



1 See De Bary ("Morphology and Biology of the Fungi, Mycetozoa, and Bacteria," 

 Oxford, 1887) and many others as to this in fungi, and Peirce (Annals of Botany, 1894) as 

 to Dodder. 



