304 
Nature and Cause of the Potato Disease. 
part, however, being cellular. The tuber has no external com- 
munication, and the skin, when set or hardened, is impervious to 
water. The cells of the tuber are its storehouses, and they are 
formed in the same way as those of the medulla of the stem ; 
they have double walls, with tubular vessels circulating between 
them, and are hexagonal in form. Beyond these parts there are 
channels in which the gems are seated ; these channels are pas- 
sages or open courses filled with starch granules, floating in the 
waters of the tuber, and surrounded by cells filled also with 
starch granules. The channels are provided for the purpose of 
feeding the gems while germinating, and they are also probably 
the course by which the products of the plant are conveyed 
through the tuber to their destination. They join the cuticle at 
its inner surface, and from it strike inwards towards the centre of 
the tuber, and are thus admirably adapted for the conveyance of 
the products and the nourishment of the germs as well. The 
tubers, although the repositories of vitality, appear destitute of it 
themselves, and can only be viewed as receptacles for the products 
of the plant. A tuber is perfectly formed when no larger than 
a pea, for, upon examination, we find it to contain all the vessels 
found in tubers of a full-grown size. We discover no imperfection 
in it, even its little cells contain starch granules. The vessels of 
the tuber are merely a prolongation of those already existing in 
the stolon, as those of the stolon are a prolongation of those already 
existing in the stem. Thus, from the vessels of the stem both the 
stolon and tuber are formed ; the stolon being the duct through 
which the products of the plant are conveyed to the tuber. 
The stolon contains all the various vessels found in the stem, 
with the exception of the spirals, and its office is simply to place 
the tuber in a drier position than the roots, and convey to it the 
products of the plant. It lives no longer than the plant works, 
for as soon as the plant ceases to elaborate, the stolon ceases to 
convey, and its functions are at an end. When the plant has 
fully performed its office, the stolons dry and shrivel up ; but 
should the powers of the plant be suddenly checked or totally 
suspended while its products are forming, the stolon, if in a damp 
situation, may putrify and decay, and thus infect the tuber to 
which it is attached. 
4. Tlie Leaves — their Office. 
The leaves perform two functions, inspiration and transpiration. 
By inspiration they receive carbonic acid from the atmosphere, 
and probably nitrogen as well. The carbonic acid thus received 
is decomposed by the plant, its carbon being assimilated and re- 
tained with the products, while the oxygen by transpiration is 
thrown off as gas to the atmosphere. The leaves also exhale 
