Metaphyta L ilium. 



149 



Prosenchyma. The prosenchyma represents parenchy- 

 matous cells which have been elongated in a fusiform 

 manner (usually) in one direction (fig. 67). Their walls 

 may become thickened either by apposition, i.e. the deposi- 

 tion of new particles of cellulose on the inner surface of the 

 cell-wall, or by intussusception, i.e. the intercalation of 

 similar particles between those of the primary cell-wall. 

 Possibly both methods of cell-wall thickening may take 

 place simultaneously. Chemical 

 changes usually take place in the 

 wall at the same time. Secondary 

 thickening may take place regu- 

 larly ; in that case we have a uni- 

 formly thickened prosenchymatous 

 cell of the type represented at fig. 

 66. In other cases we may have 

 parts of the primary cell-wall left 

 uncovered, when a pit-like depres- 

 sion results (fig. 67). In other 

 cases still we may find that the 

 secondary thickening has been laid 

 down (in this case by apposition) 

 in the form of a spiral band or 

 annular rings upon the primary 

 cell-wall. These varieties of cell- 

 wall thickenings are commonest 

 amongst the next type of tissue. 



Cell-fusions, vessels. Vessels 

 result from the fusion of two or more elongated cells 

 arranged end to end. The adjacent walls have become 

 completely broken down, or the walls may become perfo- 

 rated by a series of minute apertures through which the 

 contents of the one cell may communicate with the con- 

 tents of the other : good examples of this type of vessel 

 are found in the sieve-tubes of the phloem (fig. 68). 

 It is probable, according to recent researches, that this 



