Nature and Cause of the Potato Disease. 
321 
sented an unhealthy appearance, and the cuticle of the stem ap- 
peared sound, I have found the cellular arrangement of the 
medulla much diseased. And in those cases where the disease 
was in an advanced sta»e, so that it could be distinguished exter- 
nally, by a discoloration or decomposition of the cuticle, the 
medulla was always the most diseased, being generally putrescent. 
The disease of the stem, I have generally observed, is limited 
to the joint attacked, and seldom extends itself longitudinally to 
any distance, being confined within the space of a few inches of 
the affected part. In the same section the medulla will be much 
diseased, while the vessels and cuticle surrounding it will be quite 
sound, and if putrescence exists the medulla is always the most 
affected, the disease penetrating the medulla and leaving the 
surrounding vessels and cuticle frequently untouched. Where the 
stem has putrified and rotted off, examination proves the seat 
of the disease still to be the medulla ; for if we continue our re- 
searches by sections transversely made, we come to parts in which 
we find the medulla much diseased, and yet surrounded by sets 
of vessels and cuticle that are quite sound. And pursuing our 
researches, we further find that the diseased parts of the medulla 
extend below the sound vessels and cuticle that surround it in 
the form of an inverted cone. We have evidence, then, in these 
facts, that the medulla is the seat of the disease so far as regards 
the stem, and if we consider the nature and mode of the circu- 
lation carried on in the plant, we shall readdy perceive why the 
leaves present the appearance they do when the plant is diseased, 
although the seat of it may be distant from them. The sap flows 
upwards from the roots through the medulla and vascular system, 
as has been noticed in the vegetation of a germ, in section 2, 
page 303. It then passes through the leaves, is acted on by light, 
and becomes the cambium or prepared sap containing the pro- 
ducts of the plant. Now as the cellular arrangement of the 
medulla is necessary to the circulation of the plant, its integrity 
is essential to a healthy action ; and if its cells become disor- 
ganised, so that a full and proper supply of sap cannot reach the 
leaves, they droop for want of nourishment ; and if the disease of 
the medulla spreads, so that the whole of its organism is destroyed, 
all the parts above where it is affected are cut off from communion 
with those below, and although the plant may exist for a time, 
yet so far as the elaboration of its products is concerned, it is dead. 
If the seat of disease in the stem be below the leaves, which has 
universally been the case, the whole of the foliage dies and pre- 
sents the appearance that has already been described. There is 
nothing however peculiar in the colour and appearance of the 
leaves of a diseased plant beyond what is observable in prelimi- 
nary stages of putrescence : for if the foliage of a healthy potato- 
