DISEASED CONDITION OF PLANTS 53 
tion with nicotine and hydrocyanic acid gas under 
unsuitable conditions causes discoloration and scorch 
of certain flowers and plants. The presence of such 
vapours as paraffin and petrol produces similar injurious 
effects. 
Diseases of Unknown Origin 
Several diseases of glasshouse plants exist which 
cannot be attributed to definite causes. Among these 
may be mentioned tulip blindness, or the failure of 
apparently normal bulbs to produce flowers. Tulip 
bulbs which have bloomed well one year may be blind 
the next and bloom again the year after. The hyacinth 
and other bulbous plants frequently develop a disease 
called “ gummosis,” in which a white gum is produced 
by the sub-epidermal tissues. The gum-bearing cells 
swell, and finally the epidermis ruptures and the gum 
exudes. The “ yellow stripe” disease of the daffodil is 
another of the same category. In an advanced stage 
the leaves develop parallel yellow bands coinciding with 
the veins, and in bad cases the leaves wither and the 
blossoms die prematurely. Another such disease is the 
failure of rosebuds to open. The buds develop normally, 
when suddenly the outer petals turn yellow and cease 
to grow. Such buds may open partially but never reach 
normal development, and are distorted. 
