84 



Vegetable Physiology. 



had been thus separated into pieces. A third variety displays 

 the spiral fibre continuous in some parts, in others branched 

 and sometimes has a netted or barred appearance, as in Figure 

 G,f. Thisis named a reticulated duct. The structure of the annu- 

 lar duct much resembles that of the windpipe of most animal* 

 which like it, is composed of membranous walls, preserved in 

 their cylindrical form by regular rings of cartilage ; and the half 

 spiral, half annular duct is constructed precisely like the wind- 

 pipe of an animal of the whale tribe, in which is a spiral carti- 

 lage terminating at intervals in rings. There are several other 

 varieties of the duct, presenting various proportions of mem- 

 brane and fibre, combined in numerous different forms. The 

 office of all these is the same, — that of conveying fluid, while 

 the true spiral vessel only contains air. The combination of 

 woody fibre, spiral vessels and ducts which constitutes vascu- 

 lar tissue is shown in Figure 6, a ; and b represents the joining 

 together of two ducts. 



Fig. 6. 



b c d e f g 



Besides these elementary organs, properly so called, there 

 are various cavities resulting from their connexion or separa- 

 tion, which require some notice. It has been already stated 

 that the cellules often leave between themselves vacant spaces, 

 to which the name of intercellular passages is given. These 

 vary in size, being largest in the most succulent plants, and 

 always contain a fluid. Besides these, there are cavities in the 

 tissue, bounded by its cellules, which have no lining mem- 

 brane, do not communicate with the intercellular passages, 

 and contain air, for which reason they are called air-cells. 

 Sometimes the intercellular passages are unusually dilated by 

 the fluids which they contain ; or by the pressure of the fluid, 

 cavities are formed in the cellular tissue. Such cavities, filled 

 with the peculiar juices of the plant, are by some named p-oper 



