CELLULAR TISSUE. 23 



cellules, is found nearly free, and they collectively form 

 a species of parenchyma. 



Where the cellules are lightly or partially united, 

 they may be found disunited entirely or in part ; as is 

 seen, for example, in the loose tissue of the leaves of 

 several Monocotyledons. 



If any cause, altering the ordinary state of vegetables, 

 should chance to break the adherence of the cellules^ 

 some may then be found disunited, and having the 

 appearances of little vesicles ; as, for example, at the 

 instant of the development of the adventitious roots, 

 and perhaps in the efflorescence of lichens. 



In numberless cases where the cellules are intimately 

 united, there are often found between them empty 

 spaces, which are the mtercellular passages ; to which 

 we shall return presently. 



Finally, there are cases where the union is so inti- 

 mate that it cannot be perceived : this is what happens 

 particularly in the cellules of cryptogamic plants, among 

 which the intercellular passages are not visible, and the 

 partitions which separate the cavities appear to be 

 simple. 



The Intercellular Passages or Canals, then, 

 are the spaces which exist between the cellules, and 

 which have no other walls than those of the cellules. 

 Their form is most frequently that of a triangular 

 prism : they are found, according to Kieser, hexagonal 

 or even dodecahedral, according to the number of the 

 walls of the cellules which concur in their formation. 

 These canals follow the general direction of the cellules, 

 either lengthways, which is the case most frequently, or 

 transversely, as in the medullary rays. They are often 

 full of water, sometimes of air, and appear also to 

 receive the proper juices. Their size varies much in 

 different plants ; they are generally larger in those with 



