A Vascular Sporangium. 61 
1 hese tracheae are fairly thin walled and exhibit well-marked 
pittings of a reticulated type. It has been possible to recognise 
five groups of these tracheae adjacent to the sporangial wall and 
distributed at intervals, as represented by/, t in photograph 1. The 
largest number found in any group was six (/ 6 )—close to the broken 
middle point of the thin side of the sporangium ( g ), whilst against 
the annulus at / 3 the number was three, elsewhere generally fewer 
than three (the number in each case is indicated by the figure 
following the reference letter /). 
Owing to their slight displacement most of the tracheae are 
seen a little obliquely, a fortunate circumstance, as it permits of the 
details of their thickening being studied. The high power 
photographs 2 and 3 and figs. 4, 5 and 6 give these details. 
Photo. 2 and fig. 4 represent the group of tracheal elements abutting 
on the fracture in the thin part of the wall (g), fig. 5, a group 
from the opposite side of the sporangium with a portion of the 
indurated sporangial wall ( w ). The oval pits are numerous and 
shew a very delicate bordering. The largest element (/ in fig. 4) 
has a diameter of .045 mm. The tracheal elements were 
evidently delicate stuctures and it is not improbable that they were 
originally more numerous than is actually indicated by the number 
of those preserved in this specimen. At places they occur quite 
flattened against the wall (as at t, fig. 6), so that they may have 
formed during life a continuous lining to the sporangium. I 
attribute their preservation in this case to the early mineralisation 
of an immature sporangium. As the block of spores ripened and 
distended the sporangium the tracheae would be unrecognisably 
crushed against the wall. It is not possible to say definitely from 
inspection of this specimen whether the elements were tracheides 
or vessels, as they are shown only as short segments. 
Though the position occupied by these tracheal elements 
corresponds with that usually associated with a tapetum, it is not 
possible to make any statement as to their origin, owing to the 
relatively advanced developmental stage of this sporangium. Were 
the data available, it would be interesting to know with what layers 
of the sporangium the tracheal zone is associated in development. 
Whether, like the endothecium of an Angiospermic anther, it arises 
in common with the tapetum, or whether some other relation 
obtains. In view of the unexpected complexity of the sporangium 
here described, attention might perhaps be profitably devoted to 
such immature Botryopteridean and other sporangia as may be 
available. 
