Nature and Cause of the Potato Disease. 
303 
to which they belon"^. They represent a cone, the base being 
in the tuber and the apex at the eye. As germination proceeds, 
the whole of the vessels elongate themselves, and the bud or eye 
advances through the epidermis, and appears externally on the 
tuber. A longitudinal section therefore shows what appears 
two or more sets of vessels, a consequence arising from their 
circular arrangement around the cellular tissue of the medulla. 
Although for convenience the vessels are represented in the 
plate as passing by the stolons without divergirfg so as to com- 
municate with them, this is not so in the plant; for the medulla 
and all the vessels except the spirals throw off branches, that 
lengthen and become the vessels of the stolons themselves ; and 
thus a communication from the medulla itself exists with the 
stolons. When the vitality of a germ is brought into action, the 
starch granules flow, apparently unchanged, up the medulla of 
the stem from the channel b, in the old set, and nourish the 
young plant. These starch granules may be traced from the 
old set through the medulla and vascular system of the stem into 
the stolons, and the flow continues from the set until leaves are 
formed by the young plant. When the plant has thrown out 
leaves, the starch granules from the set cease to flow^ and can no 
longer be traced in the stem from the set. The lowermost stolon, 
1 A, is formed before 2 a : but 2 a will have tubers formed before 
1 A shows any appearance of them. The formation of the stolons 
and tubers is therefore inverted, ?nd seems to be governed by 
proximity of source supplied from : but whether this peculiarity 
in the mode of secretion be the consequence of electrical action, 
or whether it be due to exosmose and endosmose, gravitation, or 
any other thing, we have no evidence to show. The crimson 
spots. A, and the vessels traversing the cuticle of the stem into 
the stolon, 2 a, plate 6, show the situation of the disease in the 
old tuber, and how it has infected the young plant. The stolon, 
1 A, is uncontaminated. 
The various kinds of vessels found in the stem are six in 
number. They are all seen in plate 6, which shows their natural 
position and arrangement. Those marked 1, plate 6, form the 
cuticle ; 2, perforated vessels with hexagonal perforations ; 3, 
vascular system, with diaphragms; 4, annular vessels; 5, spirals; 
and B 6, the medulla, or cellular arrangement. The cuticle, 
fig. 1, is composed of horizontal tubes woven together by a fibrous 
net-work. It is firmer in its texture and character than the other 
parts of the stem, and, like the leaves, is protected by hairs that 
terminate in a sharp point. 
3, Construction of the Tubers and Stolons. 
The tuber is composed of bark, cuticle, and cells, the chief 
