280 Studies in Forestry [CHAP. xin. 



generation, demands special mention. It not infrequently 

 happens that the spores of one fungus do not produce an 

 individual resembling the parent, but something which appears 

 quite a different kind of fungus ; and the spores of this again 

 may perhaps produce what seems a third kind of fungus not 

 resembling either of the two previous forms ; but ultimately 

 the spores of the second or third forms produce, or throw back 

 to, the original parent type, and the cycle re-commences. 

 Accurate observations have thus shown, and are still showing, 

 that many fungi previously reckoned as belonging to different 

 genera are merely different stages in the development of other 

 well-known species. Those genera are of course most highly 

 organized, in which sexual processes can be proved. The 

 regular change of form is called change of generation ; the change 

 in the shape of the regenerative organ is known as pleomorphy; 

 and the change in the choice of a host, i. e. of the plant 

 infected, is termed heteroecy. This change of generation finds 

 its complete parallel among certain gall- wasps (Cynipidae] in 

 the insect world. Thus Cynips aptera, producing galls on Oak- 

 roots, gives birth to Cynips terminalis^ forming galls at the ends 

 of the twigs, whose progeny throws back to the original form 

 C. aptera, and the cycle commences anew. 



The occurrence and the spread of fungal diseases are specially 

 favoured by certain conditions of climate and situation. Damp- 

 ness and warmth are requisite essentials, whilst light is of less 

 importance, as their peculiar processes of assimilation proceed 

 without the agency of chlorophyll, inasmuch as they cannot, 

 like green plants, assimilate carbonic acid; hence, if light were 

 essential, they could not grow underground, and in the inside 

 of timber trees, as they do. In damp seasons, and in humid 

 localities with heavy, stagnant air, fungoid growth is more 

 favoured than in dry years, or in exposed situations with free 

 play of winds. Nitrogenous substance favours their growth, 

 as the plasmic contents of the mycelial threads contain a large 

 proportion of Albumen. In timber-work, the quantity of nitro- 



