Nature and Cause of the Potato Disease. 
309 
absence be long continued it suffers in proportion. In short, it 
may be said that light is as essential to the well-being of a plant 
as water, air, and even the integrity of its own organism, for 
without its presence that organism cannot long act. Deprive a 
plant therefore of light, and you rob it of a stimulus that nothing 
can supply, and without which it cannot healthily exist. From 
these brief considerations of light, which we shall have occasion 
hereafter to refer to and enlarge upon, I shall pass to the next 
part of our subject, which is — 
7. Tlie Fungi and Insects observable in Diseased Plants, with 
some Remarks on their Nature and Office- 
The fungi which I have observed in diseased plants and tubers 
are as follows : — 
1. A short white fibrous fungus that lies close to the surface of 
the tuber on which it grows, such as is represented at the base of 
the boleti, plate 7, fig. 1 A. This kind is seldom found in very 
putrescent tubers, as much moisture destroys it. It thrives best 
on slightly moist surfaces in a dark place. 
2. The round-headed or boleti kind that grows on a long 
slender tube, such as is represented in plate 7, figs. 1 B and 3. 
This kind I have only found where putrescence exists : moisture 
and absence of light appear essential to its existence. 
3. Radiated fungi, such as is seen in plate 7, figs. 2 and 4. 
4. Botritis infestans, a grape-like fungi. See plate 8. 
Plate 7. 
Mould, Fungi, &c.— Power of object-glass iised, 480 diameters. 
