sa 



BOTANY 



as the typical form, of the vessels of tracheary tissue. 

 Ringed and reticulated vessels are opposite modifications 

 of the spiral form ; the first are due to an under-develop- 

 ment of the thickening in the young vessels, resulting in 

 the production here and there of isolated rings (Pig. 

 30, v) ; reticulated vessels are due, on the contrary, to an 

 over-development, which gives rise to a complex branch- 

 ing and anastomosing of the spirals (Fig. 

 20, v'""). 



52. (3) Scalariform Vessels. — These are 

 prismatic vessels whose walls are thickened 

 in such a way as to form transverse ridges. 

 They are wide in transverse diameter, and 

 their extremities are fusiform or truncate 

 (Pig. 31). 



53. (3) Pitted Vessels.— Th& walls of these 



Fig. 21. Fio. 22. 



FiQ. 21.— Scalariform vessels of the common Brake (Pteria'). 



FiG-.p-^P'**«"i J<=ssels of Dutchman's-pipe <Aristolochia'sipho1 frnm 

 a longitudinal section of the stem; the vessel on the right is seen in seJ? 

 tion, that on the left from without, a, a, rings, which are rlmSants of 

 the original transverse partitions; b, 6, sections of the walls '*'™'^'^"^= °i 



