THE ANGIOSPERIVIAE : STEMS 



86i 



of callose may then develop on both sides of the plate, which normally ends 

 the active life of the sieve tube, though there is evidence that in some plants, 

 like Vitis, the callose pad may form temporarily during the first winter and 

 then be dissolved in the spring, only to reform permanently in the second 

 winter. 



Where sieve tubes are in lateral contact there are often connections 

 between them, formed on the longitudinal walls. They are called sieve 

 fields and seem to be closely similar to sieve plates in structure, though 

 smaller and less clear. 



The walls of the sieve tubes are of cellulose, but in a highly hydrated 

 state. When mature the wall is thick and soft and has a distinctive pearly 

 lustre, like collenchyma. 



Summary of Elementary Structure 



Stems always bear leaves, either green foliage leaves, modified floral 

 leaves or reduced scale leaves. These are attached to the stem at joints or 

 nodes and are separated by lengths of bare stem called internodes. The 

 leaves develop at the apex of the stem, 

 arising as little cushions of cells on 

 the sides of the dome-shaped apical 

 meristem (Fig. 845). 



In the apical meristem vascular tis- 

 sues develop as strands which enter the 

 base of each developing leaf. These are 

 called vascular bundles, and they are 

 also leaf traces, for every bundle sup- 

 plies a leaf. Some distance below the 

 leaf base, usually at least two inter- 

 nodes below and sometimes more, the 

 leaf trace bundles are attached to older 

 bundles, so that they form part of a 

 continuous system. 



Vascular bundles consist of xylem 

 on their inner sides and phloem on 

 their outer sides (Figs. 846 and 847). 

 Between the two lies a narrow belt of 

 cambium, whose cells are tangen- 

 tially flattened and arranged in radial 

 rows. These cells repeatedly divide 

 tangentially, producing cells which 

 develop into xylem cells or phloem 



cells according to the side of the cam- ^^^^ 84S.-Diagram of a theoretical plant 

 bium on which thev lie. The vascular form showing the relationships of the 



tissues of the bundles thus continue to various principal .f/^^^^r^:' , .'^J^^.;";- 



cular system is indicated b\ black lines. 



expand radially throughout life. (After Sachs.) 



