LECTURE YU 
VASCULAR TISSUE-SPIRAL VESSELS. 
Vascular Tissue . — 1 This tissue, bearing as it does the 
same relation to the growing portion of the plant as the 
vascular system of animals does to their general orga- 
nism, may be considered as the most important part of 
the plant. It forms no exception to the general rule 
with which we started, that all the textures originate in 
cells, for these vessels are but elongated cells joined end 
to end, and ultimately communicating with each other ; 
their walls are in most cases supported either by a com- 
plete or modified spiral fibre. These spiral vessels, 
were called by the older microscopic botanists, trachea, 
from their resemblance to the ramifying air tubes 
of insects ; nor is the analogy far fetched, since in 
either case the tube is kept patulous by an elastic spiral 
coil of fibre, which has a tendency to unrol itself when 
