INJURIES INDUCED BY PLANTS 113 



parasitic and epidemic in character, and is to be attributed to 

 Hysterium Pinastri. Where " the cast " has become a calamity 

 which year after year overtakes the seed-beds and young woods, 

 it may at once be assumed that the disease is present in this 

 most destructive form. 



It may frequently be recognized on young pine-seedlings 

 even in the first autumn by the primary leaves acquiring 

 brown blotches, while the other parts often assume a purple- 

 red colour. 



Even at this early stage one always finds the characteristic 

 mycelium of the parasite in the brown blotches. Frequently it 

 also happens in the first autumn that a large number of very 

 small black spermogonia appear on the diseased leaves (Fig. 56, 

 d, e\ the spermatia of which do not seem to be capable of germi- 

 nating. After wet summers I have found perfectly ripe apo- 

 thecia on the leaves of young pine-seedlings even in autumn. 

 As a rule the black apothecia (Fig. 57,-*), which are much larger 

 than the spermogonia, do not develop till the following year. 

 Everything depends very much on the weather. On account of 

 the dry leaves being unable to offer any nourishment to the 

 fungus, its development, and that of its sporophore, can proceed 

 only during wet weather. Dry summers and cold winters do much 

 to hinder the development and distribution of the fungus, whereas 

 wet summers and mild muggy winters are specially favour- 

 able for its growth. During mild winters the blight frequently 

 spreads rapidly in nurseries and in regenerated forest areas. I 

 have never observed the apothecia make their appearance during 

 the first year on the leaves of pines two years old and upwards. 

 They usually appear only in the third year, and generally after 

 the leaves have fallen, though they not unfrequently also ripen, on 

 leaves that have remained in situ. As regards the manner of 

 distribution of the blight-fungus, it may be mentioned that the 

 ripe apothecia rupture only after long-continued rain. Then the 

 tissues of the leaf have been thoroughly softened, and a plentiful 

 supply of water has been able to reach the apothecia from within. 

 This causes the asci and spores to swell, a state of things 

 which is followed by the forcible rupturing of the apothecium- 

 cover. Long-continued rains, however, do not usually occur 

 except with west winds. They are less frequent with north or 



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