474 Mr. VALENTINE on the Development of the Theca, 
existing between the thecal and columellar membranes. If the sporules were 
developed in the columella, we should find them occupying this space between 
the axis and middle membrane: but, on the contrary, they are invariably con- 
fined between the middle and columellar membranes. . 
It will be easy to prove that the sporules are not formed of the columella 
by a breaking up or separation of its tissue: the only foundation for which 
opinion is, that the columella, in some species, shrinks into so small a space as 
not easily to be detected ; a fact readily accounted for when we consider that, 
in those instances in which the supposed separation takes place, the cells of 
the columella are remarkably large, and consequently formed of but little 
solid material; so that when the fluid (of which in the young state the cells 
are always full) is dried up, the tissue contracts to the bottom of the theca, 
and is there easily overlooked. I have examined the thecz of several of the 
Phascums, in which genus the columella generally shrinks very remarkably ; 
and I have always succeeded in stretching out the collapsed organ to its 
original dimensions. The columella of Gymnostomum pyriforme, in an old 
theca, occupies but a very small space compared with what it did when 
young. A section of this may easily be stretched to the full diameter, and 
then it becomes manifest there has been no dissolution of the tissue. The 
view here given is supported by the following considerations, which also show 
the probability that the cellular tissue, at least, of all plants is formed in this. 
manner. We know that the elaborated juices of Dicotyledonous plants descend, 
between the bark and the wood, in the state of a thick viscid fluid called cam- 
bium: and we know that there the alburnum and liber are formed. If a 
delicate longitudinal section of the end of a growing Hyacinth root be made, 
we shall find, at the very extremity, a soft, thick, viscid fluid covered by 
the cuticle. A little nearer the bulb are a number of minute points: still 
nearer, these points are larger and more transparent: nearer still, they 
are of a considerable size and transparent, until, gradually, they assume 
the appearance of cellular tissue. There are no vessels in this part; they 
being gradually sent damit from the bulb after the cellular tissue is 
formed. | ' > 
Lastly, the sheath of viscid fluid, which, by becoming celle connects the 
seta with the vaginula, may be cited in corroboration. "The cellules of 
