The Life of the Plant. 231 
has not yet been sufficiently investigated, is one of con- 
siderable practical importance, the deet-disease. It shows 
itself sometimes in the winter, its presence being manifested 
by the root becoming light-brown and transparent from — 
without inwards. 
‘The diseases to which bulbs are subject have been but 
little studied. In the ring-disease, called also the hya- 
cinth-pest, rust-coloured spots spread from the disc over the 
scales, advancing from below upwards. ‘The innermost 
scales are first attacked, the leaves turn yellow, the flower- 
stalk decays, and finally the whole bulb degenerates into a 
muddy mass. The so-called zwAzte smut consists in the 
bulbs rotting from above downwards under the influence of 
a white slime. : | 
Besides the diseases already named, there are many 
others of less practical importance caused by parasites. Of 
these causes of disease, phanerogamic parasites are in gene- | 
ral less injurious than cryptogamic. Of the former there | 
are only a very few natives of Europe, the mistletoe (Viscum 
album), Loranthus europeus, Lathrea squamaria, the dod- 
ders (species of Cuscuta), the various species of Orobanche, 
Monotropa Hypopitys, and Thestum humifusum. | 
The mistletoe usually grows on the apple, but also on 
other trees. It weakens the trees which it attacks by the 
penetration of its sucking-roots (Fig. 378 1.), and causes the » 
formation of knots, sometimes killing the branch on which 
it grows, or even the whole tree. 
The various species of Cuscuta (dodder) germinate in 
the ground, and only subsequently strike their roots into the 
neighbouring ‘host ;’ but they also develope above ground 
their ‘ haustoria’ or sucking-roots at those spots where they. 
come into contact with the plants which they embrace. C. 
europea grows readily on hedges and thickets, and attacks 
the hop, [stinging-nettle], hemp, and other herbaceous [and 
shrubby] plants. C. ezéénum is exceedingly destructive to 
flax. C. Zrtfoli (Fig. 378 11.) is especially injurious to fields 
