CHAP. II. 
SCURF. 
63 
smooth surface and an uneven interior membrane. This is 
easily observed in the jointed hairs of Tradescantia virginica, 
where the nature of the current is distinctly shown by 
minute molecules, that are carried along by the stream. 
If the hair of Tradescantia is suffered to die on the field of 
the microscope, and dry up, it then becomes evident that it 
is composed of two sacs, the one firm and external, tlie 
other extremely thin, and after death contracting so much as 
to leave a considerable space between its sides and the ex- 
ternal sac. (See Plate II. fig. 14. h.) It appears to me that this 
is the general structure of all hairs in which a circulation of 
sap, and the cytoblast are both visible ; and it is probable 
that the external sac is the cuticular membrane, hard, firm, 
and scarcely capable of shrivelling; while the internal sac is a 
cell of the parenchyma, thin-sided, and not acquiring any 
firmness with age, but shrivelling up as soon as the fluid 
which distends it when alive is withdrawn. 
4. Of Scurf. 
Scurf consists of thin flat membranous disks, with a ragged 
margin, formed of cellular tissue, springing from the epi- 
dermis. It may be considered as a modification of hairs ; for 
it differs from those bodies only in being more compound. It 
is of two kinds. Scurf, properly so called, and Ramenta. 
Scurf, properly so called, are the small, roundish, flattened, 
particles which give a leprous appearance to the surface of 
certain plants, as the Elaeagnus and the Pine Apple. (Plate 
I. fig. 10. h.) They consist of a thin transparent membrane, 
attached by its middle, and, owing to the imperfect union, 
towards its circumference, of the cellular tissue of which it is 
composed, having a lacerated irregular margin. A scale of 
this nature is, in Latin, called lepis, and a surface covered by 
such scales lepidotus — not squamosus, which is only applied to 
a surface covered with the rudiments of leaves. Scurfs are the 
poils en ecusson (pili scutati) of De Candolle. 
Ramenta [VaginellcE) are thin, brown, foliaceous scales, 
appearing sometimes in great abundance upon young shoots. 
