FIBRO VASCULAR TISSUES: VESSELS 



of the oak. In Figs. 8ia and b is shown the phenomenon of tylosis 

 in the vessels of the white oak. In this species all the larger vessels 

 are more or less completely occluded 

 by tyloses extending from and only 

 excluding the latest complete annual 

 ring. In c and d a similar condition is 

 represented in the case of Quercus 

 Engelmanni. In this instance the 

 tyloses have developed a very thick and 

 strongly pitted wall, a condition more 

 rarely found. In e and / tylosis as 

 found in the case of the black locust, 

 Robinia Pseudacacia, is represented. 

 Here the ingrowths derived from adjoin- 

 ing parenchyma cells are very numer- 

 ous and form a mass completely 

 occluding the cavity of the vessel. In 

 Fig. 82 are seen, both in transverse 

 view, earlier stages of the invasion of a 

 vessel in the oak through the agency of 

 tyloses. The representation in this 



FIG. 82. Diagrammatic 

 representation of the process 

 of tylosis. 



illustration is somewhat diagram- 

 matic to show more clearly the 

 entry of the ingrowths through 

 the medium of the pits in the 

 walls of the vessels. 

 The phenomenon of tylosis is in all probability an extremely 

 ancient one for the higher plants, as it is found far back into the 



FIG. 83. Tylosis in tracheids of 



Liquidambar . 



