8 
Review . 
sponding structures in the leaves of the sporophytes, or be regarded 
as relegated to the axis when the leaves of an originally leaf-bearing 
sporogonium disappeared. The latter hypothesis, to which Mr. 
Church inclines, seems gratuitous. Why should the leaves have 
disappeared completely leaving no trace? Is not the hypothesis 
of independent origin of the photosynthetic mechanism both 
simpler and fully justified on the general principle of homoplasy ? 
The “immersed” archesporium of the sporogonium (and 
equally of the sporangia of Pteridophyta) is to be taken as a layer 
of potential tetrasporangia, a tetrad being the characteristic 
limiting term of the benthic unilocular sporangium in which the 
meiotic mechanism determines the number four. Such immersion 
is commonly met with in the stichidia of the bulkier Florideae, and 
the later complications of the central portion of the axis (columella) 
and of the peripheral “ wall ” found in the larger sporogonia of the 
mosses, are merely further modifications, while degeneration is 
illustrated in the smaller forms culminating in the simple 
sporogonium of Riccia. In the Pteridophytes “the view of a 
continuous stratum of archesporial cells being normal for older 
fern-laminae, followed by the progression of sorus-restriction, and 
the localization of smaller patches to a system of more elevated 
out-growths (sporangia), as secondary emendations for wind- 
dispersal,” forms part of Bower’s account of the evolution of the 
sporangium. But the aerial wastage of spores is far greater than 
the marine wastage—Mr. Church estimates it in Aspidium at 
about 1000 times that of the Laminarian—and to meet this the 
spore-producing surface increases vastly in bulk, culminating in 
the tree ferns ( Alsophila , up to 60 feet high). 
This enormous wastage is quite overcome in the heterosporous 
forms, where spore production is brought into immediate relation 
with the production of gametes already economised by the 
archegonial mechanism; and the prothallus, which through its 
precocious juvenile sexuality is already far more efficient than the 
large sexual plant as a means of obtaining quick returns in the 
reproduction of the race, is still further economised, and the 
necessary duration of its life shortened, by inclusion in the spore. 
The culmination of this process in the seed plants, where there is 
no wastage at all of sexual cells as such, and a minimum of wastage 
of microspores (pollen grains) and thus of male gametes when the 
insect pollination mechanism is brought to the greate pitch of 
perfection, is familiar ground. 
