ANATOMY OF TISSUES. 61 
position of phloem and xylem. Before these are described the 
separate elements will be considered. 
Beginning with the phloem, the sieve-tubes — or cribrose 
tissue as they are called 
are its most important element. 
They were discovered in 1837 by Theodore Hartig, and after- 
wards studied by von Mohl, Naegeli, Hanstein, and others. 
They are found in all vascular plants but are most highly 
developed in the angiosperms. Here they originate, for the 
most part, from the cambium cells and at first are similar to 
these in shape and size. When fully grown, they vary much 
in size, especially in length. In certain climbing plants they 
sometimes reach the length of two millimeters. Their maxi- 
mum width, which is from .02 to .08 of a millimeter, occurs in 
these plants. 
All the walls of the sieve-tubes are thin and consist mostly 
of pure cellulose. The transverse walls are either horizontal, 
that is, at right angles to the long axis of the tube, or oblique. 
On these walls occur the formations known as sieve-plates or 
fields, which give the name to this tissue. These are described 
as circumscribed portions of the wall, somewhat thinner than 
the remaining part, and containing numerous small openings or 
pores. ‘The pores are roundish or hexagonal in form, and sep- 
arated from each other by narrow bands of membrane. On the 
horizontal cross-walls, the plates occupy nearly the whole space ; 
and as the openings are comparatively small, the reason for the 
name sieve-plate is quite apparent. The oblique walls, whose 
surfaces are necessarily longer than those of the horizontal, 
usually contain several of these plates, which are oblong in 
form, crossing the wall so as to lie one above another, and sep- 
arated by narrow bands of membrane. ‘This is known as the 
ladder or gridiron arrangement. 
Many sieve-plates retain throughout life the simple structure 
described above ; others alter by assuming the condition termed 
by Hanstein, callous. The change consists in the thickening 
