ROOTS AND THEIR FUNCTIONS 75 
stem. But in the young root the xylem and 
phloém are arranged alternately, whilst in the 
stem and leaf they are superposed in pairs, 
with the phloém usually exterior in position. 
The cells of which the woody or xylem 
portion of the vascular strands is composed 
generally undergo a peculiar change in chemi- 
cal and physical characters known as lignifi- 
cation. Lignified walls are less extensible 
and less collapsible, and in general are more 
rigid than the ordinary cellulose membranes. 
Moreover the lignified walls often become 
considerably thickened, which further empha- 
sises the same qualities. 
In dealing with wood, especially in the 
stem, we must remember that we are con- 
cerned with a complicated mass of tissues 
associated with the discharge of many and 
very different functions (Fig.11). Some of the 
wood tissues are concerned with storage of 
food, others have to do with the mechanical 
functions of support, etc. To these we shall 
return later, but the special tissues of xylem 
that just now concern us are those which 
are connected with the conduction of water. 
The cells of the water-conducting tissue differ 
amongst themselves in details, but they are 
commonly elongated in form, and are arranged 
more or less in longitudinal continuity. It 
sometimes happens that the end walls separa- 
ting two or more cells become perforated or 
even obliterated, so that the cavity of one 
