398 ROYAL SOCIETY OF CANADA 
The number of thylosal groups present in a vascular bundle in this 
form, varies with the shape of the bundle, from one to several. The 
position of the canal and thyloses also varies with the shape. In the long 
or only slightly curved bundles the canals are formed at various points 
on the inner edge of the xylem band (Photo 20), and thus border on 
the xylem only on one side. In horse-shoe or crescent-shaped bundles 
the canal is formed right in the bend of the xylem, and is thus surrounded 
by xylem on all sides, except where the crescent opens out to the grouni 
tissue. Lastly there are bundles where the protoxylem lies in the centre, 
and when it becomes disorganised the canal that is formed is in the 
centre of the xylem, and surrounded by it on all sides. In this case, if 
there happen to be any parenchyma cells present, thyloses are formed. 
but if they are not present, the canal remains unblocked. 
The above forms all belong to the Filices or Ferns proper, and show 
the prevalence of thyloses in that group. The Ophioglossee will now 
be dealt with, Botrychium virginianum being the form described. 

Fig. 27.—Botrychium, virginianum, stipe. Thyloses. Th., in process of division 
X 490. 
Only some young stipes of Botrychium virginianum were available, 
and sections obtained from these showed a ring of bundles composing a 
vascular cylinder. The bundles are simple or compound and are separ- 
ated from one another by portions of the fundamental tissue. The 
protoxylem groups stand out as points on the inner side of the bundles, 
and in the sections examined showed breaks which represent the canals 
noted in Pteris and other forms. 
Bordering the canal, and jutting into it, were seen cells of mucn 
greater size than the adjacent parenchyma cells, but not differing very 
much in other respects. These cells are the thyloses which are here very 
rich in contents. They contain large masses of cytoplasm with very 
prominent nuclei (Photo 21). In many cases a thylose was seen to be 
in connection with a parenchymatous cell, division having already taken 
place as was evident from the thin cross-wall (Fig. 27). In all these 

