128 DISEASE IN PLANTS. 



that some influence is exerted through cells 

 themselves not directly affected. This latter 

 point need not surprise us now we know that 

 the cells of plant-tissues are connected by fine 

 protoplasmic strands passing through the 

 separating cell-walls. 



But the invading plant need not actually 

 enter the cells, and may still stimulate them 

 through both its own and their own cell-walls 

 to abnormal growth. This is well shown by 

 the intercellular mycelium of Exoacus and Exo- 

 basidiwu, and the latter affords an excellent 

 illustration of the far-reaching effects of hyphae 

 on the cells (of Vaccmiuni) into which they do 

 not penetrate. Not only are the cells stimu- 

 lated to grow larger and divide oftener than 

 normally, thus producing large gall-like swellings, 

 but the chlorophyll disappears, the cell sap 

 changes colour to red, the numerous compound 

 crystals normally found in the tissues diminish 

 in number and are different in shape, large 

 quantities of starch are stored up, and even the 

 vascular bundles are altered in character. All 

 these changes indicate very profound alterations in 

 the physiological working of the protoplasm of the 

 cells of the host, and yet the fungus has done its 

 work through both its own cell -walls and those of 

 the host. 



Even harmless endoph}-tic algae in the inter- 

 cellular spaces of plants may stimulate the cells in 

 their immediate neighbourhood to increased growth, 

 e.g. Anabaena in the roots of Cycads. 



