STRUCTURE OF DOTTED DUCTS. 69 



well seen, in the bundles of Vascular tissue in the petiole of the 

 leaf of Ground-Ivy and other plants. 



83. These two forms are especially interesting, from the 

 analogies which can be found to them in the Animal structure. 

 The close correspondence between the spiral vessels, and the 

 trachea or air-tubes of Insects, has been already pointed out. 

 On the other hand, the annular duct corresponds with the wind- 

 pipe of higher animals ; the membranous walls of which are pre- 

 vented from falling together, by means of rings of cartilage dis- 

 posed at regular intervals. And the half-spiral half-annular duct, 

 which is the intermediate form in plants, precisely corresponds 

 with the structure of the wind-pipe of the Dugong (one of the 

 Whale tribe), in which we find a spiral cartilage^ terminating at 

 intervals in rings. 



84. There are other forms of ducts, again, in some parts of 

 which the traces of the spiral structure are very obscure ; whilst 

 in other portions of the same tube, they can be easily distin- 

 guished. In these it appears as if the spiral fibre had been 

 broken up into small fragments, and that these had served as 

 centres round which new deposites had accumulated; so that they 

 had grown irregularly together, leaving interspaces in which the 

 membrane is uncovered (as in the dotted duct) by this secondary 

 wall. In fact it often happens that a duct, which exhibits in 

 one part distinct remains of the spiral structure, 

 approaches the character of the dotted duct so 



closely in another part, that they can scarcely be 

 distinguished ; and it is probable that the interior 

 deposite which gives to the latter its peculiar cha- 

 racter, may have originally taken place around the 

 fragments of a spiral fibre. 



85. In other instances, however, it seems clear 



that the ducts have originated in cells, which have FIG. 30. 

 lain end to end, and have been made to commu- JJJJ," D OWED 

 nicate with each other, by the breaking- down of DUCT, showing 



-''.., r that the dots are 



the partitions between them ; for the remains ol thinner spaces 

 these partitions may be not unfrequently detected. of its wall& 

 Sometimes these ducts remain, like the cells from which they 



