LEAF AND SEEDLING DISEASES 179 
willows and poplars the rust is generally very conspicuous. 
Microscopically members of the genus are distinguished by 
the large club-shaped paraphyses which grow among the 
uredospores. The uredosorus has no peridium such as is 
found in Melampsoridium. 
Like Melampsoridium, Melampsora causes very little 
damage to the larch, except where it becomes epidemic in 
a nursery (see W. G. Smith, 1886). When found it will be 
difficult to identify, and if the disease is sufficiently harmful 
to justify special measures, it will be well, as far as possible, 
to remove poplars and willows from the neighbourhood of 
the nursery. 
The damping-off of larch seedlings. There are two known 
fungi which cause the damping-off of larch seedlings. It is 
not easy to distinguish between the two diseases with the 
unaided eye, but the conidiophores which grow from the 
diseased portions are readily identified under the micro- 
scope. Fig. 73 shows the conidia of the two species side 
by side. 
Phytophthora omnivora, de Bary (=Ph. fagi, R. Hartig), 
attacks the seedlings of all conifers and of the beech. It is 
very closely allied to Phytophthora infestans, the fungus 
that causes the best known potato disease. The life-history 
of the fungus is somewhat complex, and will only be briefly 
described. The conidia shown in fig. 73, A, are borne on 
conidiophores which either grow through the stomata of 
attacked portions of the seedling or break through the 
epidermal wall and cuticle. The conidia fall on to the 
ground and, instead of germinating in the usual way, the 
contents divide up into a number of parts, each of which 
becomes a swarm-cell, which escapes and swims about in . 
the films of water contained in the soil. For a cell which 
produces these swarm-cells the word ‘ conidium ’ is a mis- 
nomer, and the term sporangium is generally used, and will 
be employed below. As the swarm-spore is known to 
botanists as a zoospore or zooconidium, the sporangium is 
also called a zoosporangium or zooconidangium. If a 
swarm-spore, while swimming about, comes into contact 
N 2 
