ROOTS AND THEIR FUNCTIONS 75 



stem. But in the young root the xylem and 

 phloem are arranged alternately, whilst in the 

 stem and leaf they are superposed in pairs, 

 with the phloem 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 



