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 thecse 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, tliick, 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 downwards from the bulb after the cellular tissue is 

 formed. 



Lastly, the sheath of viscid fluid, which, by becoming cellular, connects the 

 seta with the vaginula, may be cited in corroboration. The cellules of 



