44 



PLANT LIFE. 



to their hosts in various ways. Sometimes the young hypha, 

 growing from a special reproductive body (spore),* so minute 

 that it may easily float in the air and fall upon a leaf, creeps 

 along the surface till it finds one of the 

 microscopic openings in the skin of the 

 leaf, into which it grows (sp, fig. 51). 

 These external openings are connected 

 with irregular spaces between most of 

 the cells of the softer parts, which are 

 also the parts in which the food-supply 

 is most abundant. In these, therefore, 

 the fungus develops, breaking out to 

 the surface again to form or set free its 

 reproductive bodies. 



Or, the young hyphae may excrete 

 at their tips a substance which so soft- 

 ens or dissolves the cell-walls of the 

 host that they penetrate these cells 

 readily, not only at the surface (sp' f 

 sp", fig. 51), but in the interior. f They 

 then branch freely, often growing in 

 the spaces between the cells, often 

 passing through the cells themselves 



Fig. 52 — Hyphae of Tra- r ° ° 



metes Pini perforating the (ficr C2). 

 walls of a wood cell (at c) of V o' D )' 



Scotch pine and destroying Plants are often attacked when mere 



the primary wall of the cell. 



d y e, holes made by hyphae. seedlings. Either from a bit of my- 



Magnined about 800 diam. ° J 



—After r. Hartig. celium or a spore which has survived 



the winter or the dry season, a hypha grows, which, almost 

 as soon as the seedling emerges from the seed, penetrates it. 

 The fungus, in these cases, may develop quickly and kill the 



* See \ 304 and the following. 



f It is not improbable that the penetration of cell-walls is assisted by 

 such pressure as the growing hypha can exert, but the relative action of 

 enzymes and pressure has not been determined. 



