316 Journal of Agricultural Research vol. xx, No. 4 



the host wall does not always yield until the advancing cell is in contact 

 with it, suggesting that mechanical pressure may here be a factor. 



The enlargement of cells under the stimulus of the fungus is the smaller 

 factor in the production of galls. As soon as the fungus has begun its 

 advance into the tissue, cell division is stimulated in the vicinity, and 

 even at a considerable distance if the fungus is making rapid growth. 

 The first notable divisions take place in the cells just beneath the epi- 

 dermis in the region of the point of invasion (PI. 55). Walls are inserted 

 tangentially to the outer surface of the structure, and the increase in 

 tissue at this point surrounds and may even bury deeply the base of the 

 fungus cavity so that it no longer leads to the exterior of the gall. The 

 thin-walled parenchyma in which the fungus forms its cavities may 

 show little morphological change near the invader in the early stages of 

 its progress, especially if these cells have matured and are not readily 

 capable of division. However, the older part of the surrounding wall of 

 the fungus cavity is soon greatly thickened with a layer which is very 

 brittle when cut and which is therefore poorly preserved in stained 

 preparations. The peculiar structure and markings sometimes found in 

 these walls has been noted by Magnus (20), though his assumption that 

 the window-like openings between fungus cavities are due to the local 

 absorption of these walls seems less probable than that they are the 

 partly filled openings through which the fungus advanced at an earlier 

 stage. As soon as this thickening is well under way, the host cells 

 adjoining the cavity begin to divide with walls tending to be oriented 

 tangentially to the wall of the cavity. Such divisions proceed further in 

 the vicinity of vascular bundles than elsewhere, giving rise to a con- 

 siderable mass of cells in parallel rows, almost cubical in shape, with walls 

 a little thicker than those of the normal parenchyma (PI. 56, A). But 

 these processes are rarely rapid enough to surround the newer portions 

 of the cavity where the fungus is slowly breaking into cells and extending 

 its ramifying maze. Perhaps the larger bulk of the cells that make up 

 the gall are developed from the vascular bundles where division, especially 

 in later stages in development, becomes very active. Sometimes a 

 bundle becomes much broadened, and from the active cambial region a 

 large mass of parenchyma on one side and a few leaf tracheids on the 

 other are set off. Tissue from this source is likely to be richer in proto- 

 plasmic contents than that from the other sources mentioned and is more 

 extensively penetrated by the advancing fungus. Thus, it may be said 

 that the response of the cells to the stimulation of the fungus is in pro- 

 portion to their capability for meristematic activity and to their nearness 

 to the source of stimulation. Cells near the exterior of the gall divide 

 with walls tangential to the surface of the gall; those in close proximity 

 to the older portions of the fungus cavity divide with walls tangential 

 to the wall of the cavity; while vascular bundles function in division 



