46 Farmer . — On Isoetes lacustris, L. 
numerous in plants otherwise widely distinct, that it appears 
scarcely legitimate to regard them as affording any other but 
confirmatory evidence in establishing a theory of relationship 
which has been already arrived at on broader grounds. It is 
of course well known that outgrowths of a somewhat analo- 
gous nature are found to be often remarkably constant in 
certain circles of affinity, such for instance as the stipules of 
Rosaceae and of Cupuliferae. But this fact really strengthens 
the case against the employment of the ligule of Isoetes as 
affording any evidence of affinity with Selaginella, for no one 
will refuse to admit that the relationship between the two is 
at the best but very remote, so different are they in all other 
important characters ; to endeavour therefore to unite them 
on account of the presence of a ligule in each of them, even if 
this structure were more similar in the two plants than as a 
matter of fact is the case, is like an attempt to establish an 
affinity between the Rosaceae and Cupuliferae on the ground 
that stipules are common to both orders. 
Development of the Leaf . — In describing the development of 
the leaf, it will be convenient to consider in the first place its 
growth in length. After its rudiment has become well pro- 
nounced as a flattened and conical papilla, further cell-division 
is chiefly restricted to a zone situated at or near the base of 
the young leaf. In plants in which sporangia have begun to 
be formed, the cells below the insertion of the ligule remain for 
some time merismatic, and thus space is provided for the large 
sporangium, and at the same time the zone, including the 
insertion of the ligule, is raised up. If, on the other hand, the 
leaf belongs to a plant which has not as yet begun to bear 
sporangia, the merismatic tissue is localised in the part of the 
leaf immediately above the ligule. This position is ultimately 
taken up by the merismatic tissue of all leaves, and it is 
to the activity of cell-division in this region that the greater 
part of the mature supraligular portion owes its existence. 
The cells which are thus formed differ in their subsequent 
growth, and thus differentiation is at an early period per- 
ceptible in the leaf above the meristem. Whilst the middle 
