86 MORPHOLOGY OF THE CELL. 



work. While, therefore, they may and probably do have a 

 common origin with spirals, it is not necessary to assume, nor is 

 it probable, that the}' have resulted from mechanical displace- 

 ments of them. The relative positions of the separate rings 

 may be explained in the same way as the steepness of the 

 spirals. 1 



274. Cases are met with, in which projections from the wall 

 may extend nearly or quite across the cell-cavity, somewhat after 

 the manner of beams. Such cross-beam cells or ducts are called 

 trabecular. A good example can be found in some of the tracheids 

 of the leaf of Juniperus communis. 2 



1 " The notion was extensively held that the spiral fibre could not follow 

 the expansion which the vessel underwent during its growth, and tore up into 

 fragments which were again united into rings, and thus brought about the 

 formation of annular vessels. Completely as this idea, which was a contradic- 

 tion to all observation, had been refuted by Moldenhawer, it remained a stand- 

 ing article in all phytotomical writings up to Meyen's Physiologic" (Mohl : 

 Vegetable Cell, p. 21). 



2 De Bary : Vergleichende Anatomic, p. 171. 



The following measurements of wood-cells and ducts are given by Wies- 

 ner (Die Rohstoffe des Pflanzenreiches, 1873, p. 525) :- 



Average diameter of wood-cells. 



Rhus Cotinus .............. 7.5 /*. 



Lonicera Xylosteon ... ........ 9.8 " 



Salix Caprsea ..... ........ 11.0 " 



Viburnum Lantana ............ 22.0 " 



Alnus glutinosa ............. 25.0 " 



Fraxinus excelsior ............ 28.0 " 



Average diameter of ducts. 

 Hsematoxylon Campechianum .... . . 112 /*. 



Csesalpinia Sappan . . .... 120 



Ochroma Lagopus ........... 140 



Fraxinus excelsior ............. 140 ' 



Ulmus campestris ............. 158 



Tectona grandis ............. 160 



Juglans regia .............. 220 



Carya alba ............... 248 " 



Quercus sp. . . . 200 to 300 ' 



The ducts in the foregoing examples are so large that in cross-section 

 they can easily be seen by the naked eye. The following are considerably 

 smaller : 



Tilia sp ................. 60 



Acer sp ................. 71 



Alnus sp ................ 76 



Rhus Cotinus ............. . 80 " 



Betula sp ................ 85 " 





