INTEODUCTION. 3 



tached. The spores produced differ wi h the season and locality of the 

 plant. They are differentiated into (1 summer spores, very delicate, 

 which generally produce a mycelium forthwith, and (2) winter spores, 

 very hardy, which do not develop until after a winter passed as such. 

 No general rule can be given as to the evolution of fungi. It differs 

 as much from one species to another as in that of insects. Species 

 exist of very complex forms, the evolution of which obliges them to 

 pass each cycle on very different species of plants. The suppression 

 of one of these plants in the district may lead to an instantaneous and 

 complete stoppage of the disease. These fungi are not all equally injuri- 

 ous. The following classiiication, based on their action on plants, has 

 been adopted : (a) Absolute jmrasites, {b) Wound or weakness j^arasites, 

 and (c) Facultative i^arasites. {a) The first, the most dangerous, are 

 capable of attacking healthy plants, {b) The second can only attack 

 the plant if its physiological condition is abnormal, i.e. attacked by 

 disease, if its vital energy be diminished by climatic conditions ; or if 

 hail, or frost, grub, or other cause, has made an opening in the epi- 

 dermis, enabling them to penetrate into the interior, (c) The third are 

 the fungi formerly termed saprophytic fungi as opposed to parasitic 

 fungi. Whilst the latter draw their nutriment from the living cell, the 

 former live on inanimate organic matter. It has been ascertained, 

 nevertheless, from the profound study of cryptogamic diseases that 

 saprophytic fungi may, in most instances, become dangerous parasites 

 if, through special circumstances, the plant cell may happen to con- 

 tain elements sought after by one of these fungi. Thus the penicillium 

 glaucum, the well-known green mould, which usually grows on an in- 

 animate medium, is attracted during the ripening of fruit by the sugar 

 contained in the latter, and penetrating therein causes them to rot. 

 (3) Microbe Parasites. — In the case of a microbe parasite it is especially 

 necessary to know what factors induce its development, which is closely 

 connected with the composition of the cellular substance, the tem- 

 perature, and the moisture. Plants, however, suffer less from these 

 than animals, because the reactions of vegetable plasma are acid, 

 which are less favourable to their evolution than an alkaline 

 medium. However, the daily discovery of parasitic microbes helps 

 to throw light on some of the morbid phenomena of plants, and 

 microbes appear to play a most important role in plant pathology. 



The antagonism between animate beings, as much animal as vege- 

 table, which live in the same medium does not give rise solely to very 

 weak, morbid conditions, but also to relations between beings of very 

 different species, which are advantageous to the two antagonists. 

 These associations are termed symbioses. De Bary, generalizing the 

 term symbiosis, distinguished (1) Mutualistic symbiosis or advan- 

 tageous co-operation of the two associates, and (2) Antagonistic 

 symbiosis, where the one lives at the expense of the other. Vuillemin, 

 on the other hand, calls the first association symbiosis and the second 

 antibiosis. 



I. Mutualistic Symbiosis. — Many cases of evident symbiosis occur 

 between phanerogams and cryptogams, between cryptogams and 

 microbes, and between phanerogams and microbes. Amongst these sym- 



