302 
Nature and Cause of the Potato Disease. 
actively employed in seeking water and inorganic matter from the 
soil for its sustenance. Great as this number of absorbents may 
appear, it will be found by examination to be underrated, for all 
parts of the roots abound with them. They are so minute that 
many of them cannot be discerned by the unassisted eye, and 
their web-like filaments are easily broken. Plate 5, fig, 2, b 
represents the absorbents as they appear when greatly magnified. 
Plate 5. 
1. Vessels of the gem, 2. Spon^'ioles of roots. 
Power of object-gluss used, 360 diameters. 
The Internal arrangement of the roots is simple ; there is an 
epidermis and a porous mass of tubes without diaphragms ; the 
sponges or absorbents are simply the porous tubes without the 
epidermis, and they arc probably no more than a prolongation of 
the roots caused by a more rapid growth of the tubes than the 
epidermis. This view seems correct from the circumstance of 
their enlargement and extension, as they grow and advance with 
the growth of the roots. The great absorbent power of the roots 
of the potato plant shows its fitness for a dry situation, and there- 
fore accords with what is needed for its products, the tubers. 
2. The Growth and Construction of the Stem. 
Plate 6 represents a longitudinal section of a stem attached to 
the set from which it sprung. 1 and 2 are stolons, E, the crown 
of the set ; B, the channel that contains the germ ; a, a diseased 
part of the old tuber or set ; c, the cellular structure of the set ; 
and I), the lower part or termination of the vessels of the germ 
from which the plant sprung. The vessels that form a germ are 
always in a bundle in the tuber, and situated just beneath the eye, 
