io6 THE ANATOMY OF WOODY PLANTS 



Mesozoic. It is not confined by any means to vessels and even 

 occurs rarely in the tracheids of dicotyledons. Fig. 83 shows the 

 presence of tylosis in the fiber-tracheids of the sweet gum (Liquid- 

 ambar styraciflua) . The heavily outlined bodies in the cavities of 

 certain of the fibrous elements are of the nature of tyloses. Similar 

 conditions have been found in the case of other dicotyledonous 

 woods. It has been made clear in an earlier chapter that ingrowths 

 resembling tyloses occur in the resin canals of the pine in the region 

 of the heartwood. Occlusion by means of parenchymatous inva- 

 sions is, however, not confined to the resiniferous spaces in the 

 genus mentioned. In the roots and likewise in the cone the tra- 

 cheids of the wood often show themselves occluded by ingrowing 

 parenchyma cells. Such conditions are not normally found in the 

 vegetative stem of living pines, although they are known to occur 

 in the branches of cretaceous Pityoxyla from the Eastern United 

 States. 



