26 1 



removed and the decomposed layers used; even an admixture of lime, or still 

 better, of calcium phosphate is advisable. 



I have mentioned the poor growth of plants in moor earth in a special 

 section, because I am of the opinion that a very considerable number of phe- 

 nomena of disease may be traced to the acids in the soil, — the gardener says 

 that the soil smells sour. Even those specific plants, such as Rhododendron, 

 Azalea, etc., only thrive when, as in their natural habitat, they stand in 

 fibrous earth which is easily aerated. In the moment when a mixture of 

 moor soil with more nutritive solid soils is used for potted plants, we find 

 root-decay, which is indicated by the brown edges of the leaves. I consider 

 the theory of the necessity of an admixture of moor soil in cultivating the 

 majority of our finer potted plants to be erroneous. As far as my experience 

 goes, sand can give incomparably better results as a loosening material. The 

 gardener should work with well decomposed leaf mould or compost earths 

 and add large amounts of sand. If care also is taken to have good pot 

 drainage, there will not be so many complaints about root diseases in the 

 future. 



Specking of Orchids. 



A special illustration of the advantages of the use of sphagnum, de- 

 scribed in the previous division, is found in the peculiar black spotted con- 

 dition of the leaves of epiphytic orchids. In our green houses there are 

 many leaf diseases which frequently arise from fungus infection (Gloeo- 

 sporium and Colletotrichum, Phoma, Phyllosticta, etc.). We find many cases 

 however, in which fungi take no part or occur only secondarily and among 

 these an infection should be emphasized especially which may be found in 

 Cattleya, Laelia, Dendrobium and the members of the group of the Vandeae. 



The course of the disease is explained best by the description of a special 

 case, which, occurring in Phalaenopsis amahilis var. Rimenstadiana^ , has re- 

 cently been studied more closely. All except the youngest leaves of plants 

 grown in leaf mould in pierced pots and watered with tap water were 

 spotted yellow to black. The disease advanced apparently from the older to 

 the younger leaves and manifested itself, in its early stages, by the appearance 

 of irregularly round or oval, pale, translucent spots. These were scattered 

 over the whole leaf, but usually appeared first and most abundantly at the 

 tip. When such leaves were cut off and lost water by evaporation, the spots 

 which became pale at the beginning of the attack, could be felt like w^arts 

 over the healthy leaf. These conditions changed, however, as the disease ad- 

 vanced, since the yellow spots at once took on a whitish appearance and 

 were depressed like saucers. In this it was seen that different adjacent cen- 

 tres of disease coalesced, forming connected, thin surfaces, which finally 

 turned a deep blackish brown and were enclosed like a wall by the healthy 

 tissue. After turning brown, however, the spots did not increase in size. 



1 Sorauer, Erkrankung von Phalaenopsis amabilis. Zeitschr. f. Pflanzenkrankh., 

 1904, Part V. 



