TISSUES. 



N 

 k 



24. The ducts are seen under three states ; the 

 annular (c), the reticulated (6), and 

 the dotted (c) ; arid it is probable 

 these different forms have their ori- 

 gin only in certain metamorphoses of 

 spiral vessels. 



25. Annular ducts consist of a 

 number of rings placed at intervals 

 within a membranous tube often of 

 a very considerable diameter; they 

 are common in succulent stems like 

 that of the Balsam, and are met 

 with in the root. 



26. Reticulated ducts have these rings broken up, 

 and the divided portions often anastamosing one with 

 another ; they are met with where the annular are seen, 

 and an intermediate state of them approaching to the 

 next form is seen in Ferns. 



27. Dotted ducts seem to connect vascular tissue 

 with woody fibre, and are composed of membranous 

 tubes with tranverse or crossing rings, between which 

 are placed little bodies very similar to what we see on 

 some true woody fibre. 



28. Proper ducts are thin membranous tubes, some- 



times anastomosing within each other, 



\~==z\ \/, and frequently running parallel with 



^C.-.jWii the veins of leaves, in which organs 



(C~~~\\ \\ they are most easily seen, especially 



^ '* -^ in plants having lactescent juices. 



The tube is plain. 



Besides these forms of- elementary structure, there are 

 certain pseudo ones which depend for their origin upon 

 the previous existence of one or other of those which 

 we have already mentioned. These are, intercellular sub- 

 stance, intercellular passages, air passages, air cells, and 

 receptacles of secretion. Intercellular substance is a 

 material of a very doubtful nature which intervenes 

 between the walls of the cellules of imperfectly con- 

 nected tissue, and the rest result from spaces being left 



