II2 ; PLANT STRUCTURES 
tiful structural adaptations by which it meets the 
various stresses to which it is subjected. The problem 
before the plant is to combine a minimum quantity of 
material with a maximum of strength and rigidity. 
Strands of toughened fibre, so disposed as to meet the 
stresses most advantageously, are characteristic of 
such stems. In the case of many tall annuals, such as 
the larger Umbellifere, the principle of the hollow 
column is largely employed; in proportion to the 
strength obtained, this is far more economical than a 
solid column: and economy is particularly necessary 
Cc. b. 
Fic. 19. ARRANGEMENT OF STRENGTHENING MATERIAL IN 
Root (4) AND IN STEM (b) (DIAGRAMMATIC). 
in such annual stems, where the time available for 
construction is short. Transverse partitions at 
intervals provide stiffening of the whole; and as the 
efficiency of the toughened longitudinal strands 
increases with their distance from the centre, the 
stems are often ribbed, the strands occupying the ribs, 
with softer substance between. This form of con- 
struction may be contrasted with that obtaining in the 
roots. In the latter the greatest mechanical stress is 
in the form of a longitudinal pull caused by swaying 
of the stem under wind-pressure. To meet this the 
