140 A TEXTBOOK OF GENERAL BOTANY 
composed largely of the soft tissues of the leaf bases, through 
which run the long sclerenchyma strands that are the abaca 
of commerce. The sclerenchyma can withstand considerable ten- 
sion, while the massive development of parenchyma withstands 
the compression. fe 
Mechanical tissues in roots. The roots of a plant serve to 
anchor it in the ground. They act like cables, and the princi- 
pal stress to which they are subjected is longitudinal tension, 
or pull. It is evident that when a plant is blown by the wind 
the roots on the side from which the wind is coming are 
subjected to longitudinal tension, and if they are not strong 
enough to withstand this they break and the plant is blown over. - 
The best arrangement of material to withstand this type of stress 
is in the form of a cord. In roots the vascular bundles and 
strengthening materials are usually much more centrally located 
than they are in the stem, the center being frequently oceupied — 
by thick-walled elements. Compare Figs. 113 and 187. 
MOVEMENT OF MATERIALS IN STEMS 
Food materials. Proteins dissolved in water travel in the 
sieve tubes. The direction of the movement may be either up 
or down, according to where the protein is to be used or stored. 
The carbohydrates travel in the phloém. What has been said 
of the direction of movement of proteins applies also to the 
carbohydrates. The carbohydrates move from cell to cell only 
when they are in solution .in water. | 
Carbohydrates in vessels. While carbohydrates usually move 
in the phloém, they are frequently found in considerable quan- 
tities in the vessels when food material is being transported on 
a large scale from storage regions to rapidly growing organs, 
as in deciduous trees at the beginning of the period of growth, 
when the leaves are expanding. In such cases the carbohydrates 
diffuse from the parenchyma into the vessels and are carried 
upward to the expanding foliage or inflorescences. This is con- 
spicuously true of the sugar maple, and explains why a solution 

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