66 



THE ROYAL SOCIETY OF CANADA 



if ever of ancestral occurrence here its physiological function would 

 necessitate its retention at this position of most need. However, as 

 previously noted, it is never found below the phloem. This condition 

 in thé cotyledon is at distinct variance with Jeffrey's view that trans- 



.c. 



.tt. 



.cp. 



Fig. 18. — Pinus strobus. 

 Cotyledon — long, tang, above protoxylem — x 550. 

 cp. — Centripetal xylem; e. — Endodermis ; 

 tt. — Transfusion tissue. 



fusion tissue in the pines and modern conifers represents a deteriora- 

 tion of the outer transfusional sheath which surrounded the entire 

 bundle in Cordaites and Prepinus (Figs. 1 and 3). In whatever way 

 the fossil transfusion tissue may have arisen the structure shown in the 

 vegetative leaves of modern pines obviously cannot be considered to 

 represent a deteriorated remnant of it. 



General. Several authors, as stated previously, have spoken of the 

 possible origin of transfusion tissue from the parenchymatous cells 

 of the pericycle. Worsdell (16) himself admits that "tracheids such as 

 those of the transfusion tissue may be formed anywhere, at any time 

 and from any tissue of the plant," and as an illustration refers to the 

 transfusion tissue found in the stem of Casuarina. This is a tracheary 

 tissue evidently developed from parenchyma and certainly absolutely 

 independent of centripetal xylem, since such a tissue is not existent 

 in the form. He might equally well have referred to the velamen of 

 orchids — a well-known example of a tracheary tissue derived from 

 cortical and therefore parenchymatous cells. 



The derivation therefore of transfusion tracheids from the peri- 

 cycle of the foliar vascular bundle is undoubtedly a possibility. Such 



