36 REACTION OF HOST TO PARASITIC ATTACK. 



number ; on the other hand, crystal-sacs, normally absent, are, 

 under the influence of Eocoascus alni iiicanae, formed in hyper- 

 trophied catkin-scales of alder. 



It may be here observed that calcium oxalate crystals are found in the 

 mycelium of many fungi. De Bary' found them very common, particularly 

 in the mycelium of species of Botrytis, and he remarks thereon : " it may 

 well be assumed that the oxalic acid is formed from the sugar inside the 

 living oxygen-absorbing fungus-cell, but is immediately ejected therefrom 

 by the carbon dioxide produced in respiration ; in other words, an oxida- 

 tion-fermentation takes place in the plasma of the mycelium. The oxalic 

 acid is probably separated iu combination with potassium and converted 

 into calcium oxalate, whea calcium is present in the pabulum of the 

 mycelium.'' 



g 7. EFFECT OF PARASITIC FUNGI ON THE CELL- WALL. 



The effect of the mycelial hyphae of parasitic fungi on the 

 cell- wall may be either mechanical or chemical. The intra- 

 cellular hyphae of fungi and the apices of the haustoria of 

 intercellular fungi must penetrate through the cell-walls of their 

 host, either of the epidermis, or the membranes of other cells, 

 consisting of cellulose alone, or in some state of lignification.^ 



The membranes may be simply pricked, as by a fine needle, 

 so that the opening, because of the elasticity of the cellulose, 

 closes up again after the perforating hypha has died. This 

 is the case with many Uredineae. In such cases the hypha is 

 constricted iu passing through the cell-wall and swells out again 

 in the free cell-cavity. Frequently, as in the case of Percmo- 

 spora densa, the haustorium will only cause a depression in the 

 membrane of the cell without penetrating it. 



In addition to purely mechanical perforation of the mem- 

 brane, the effect of the hyphae may also be a chemical one, so 

 that the wall is dissolved and the holes produced remain long 

 after the hyphae which made them have disappeared. This 

 solvent effect is probably always present in cases where per- 

 foration of lignified membranes takes place. It is a constant 



'De Bary. Botan. Zeitung, 1886. 



'' De Bary. Biology and Morphology of the Fungi. English Edition. 

 H. M. Ward. " On a lily-disease," Annalu of Botany, 1888. 

 Miyoshi. "Die Durchbohrung v. Membranen durch Pilze.'' '^Pringsheitn's 

 Jahrbuch, Vol. 28, 1895. 



