94 
HISTOLOGY OF VEGETABLES. 
freed from confinement within the vessel. Moreover, in 
old dry parts of the plant in which their function has 
ceased, they are found to contain, as in insects, only air, 
but in the young and growing parts they contain fluid, 
and floating raphides are found in abundance in the 
spiral vessels of the young branches or shoots of the 
common grape vine. Spiral vessels, however, differ 
from the air-vessels of insects in being generally straight 
and unbranched. 
In order to illustrate what I have been describing, 
it will be well for you to see the trachese of the 
larva of Dyticus marginalis ; in any of the large 
branches, as shown by Fig. 81 , the presence of a 
spiral fibre within a membranous tube, will be re- 
marked. For the sake of comparison, a portion of 
FIG. 81 . 
FIG. 82 . 
Portion of a tracheal tube of the larva 
of Dyticus marginalis. 
Portion of a spiral vessel of 
a plant. 
a spiral vessel of a plant is represented by Fig. 82 , 
in which it will be seen that the principal point of 
