W. C. Worsdell. 
THE PRINCIPLES OP MORPHOLOGY.—II. 
The Evolution of the Sporangium. 
N continuation of the line of thought adopted in the last 
article, I would point out that the primitive origin and point 
of departure for everything that we find in the sporophyte generation, 
from which every subsequent tissue, organ and individual plant has 
evolved, is the undifferentiated sporogonium like that of Riccia, in 
which the three morphological categories of “stem,” “leaf” and 
“ root ” find their point of fusion in one well-nigh homogeneous 
spore-producing body, which is one of the simplest of thallus- 
structures. At this point the entire plant may be said to consist of 
sporogenous tissue, enclosed within a limiting sterile layer of cells. 
The next step in differentiation was that in which, during 
vertical elongation of the organ, a foot or sucker, the homologue of 
the future root of the higher plants, became sterilised out of the 
fertile tissue. Such a stage is represented for us in the Liverwort 
Sphaerocarpus. From this point more than one diverging line of 
evolution must have proceeded; one of these gave rise to the 
modern Bryophytes in which the capsule, while remaining, as a 
rule, unbranched, became highly specialised and complex in structure, 
this complexity being a wholly secondary character. Another line, 
however, must have proceeded by way of branching, or, rather, 
duplication, of the sporogonium in the manner I described last 
time, producing thereby the earliest and most primitive vascular 
Cryptogams. The sporogonial capsule and every subsequent exact 
duplication thereof is at once the primary sporangium, and the 
primary “leaf,” possessing radial symmetry of construction; their 
fused setas constitute the axial substratum on which these primitive 
organs are borne, and is unequivocally a compound structure. The 
mode of branching of the whole would be eventually repeated in 
each lateral sporogonium, and, at the same time, sterilisation of the 
sporogenous tissue, combined with the assumption of a bilateral 
character of the primitively radial organ would take place. 
Hence it is certain that foliage-leaves are a purely secondary 
adaptation, for they must have originated by complete sterilisation 
of the primary sporangia. It is obvious that, out of these latter, any 
degree of subsequent differentiation can be postulated. 
It seems not unnatural to suppose that one of the earliest 
types of sporophyll, in which a certain amount of sterilisation had 
taken place, was that which possessed radial symmetry and a single 
