248 SUMMARY OF THE WORKING HYPOTHESIS 
intercalated as a consequence of sterilisation, and will therefore take a 
secondary place. An important question will then be how this more 
elaborate condition of the strobilus of Vascular Plants came into exist- 
ence. Any theory of the origin of the strobilus should be based upon 
detailed knowledge of its structure and development, in forms living 
‘and fossil, and of its parts: these are the axis, which is the central 
part in any strobilus; the appendages; and the sporangia, which are 
usually produced in relation to the latter. These parts will require 
separate consideration. ; 
A detailed study of the sporangia of Vascular Plants has led to the 
following definition of the sporangium (Chapter VITI.), which discards 
non-essential and fluctuating characters, and retains only what is essential 
and constant. ‘Wherever there is found in Vascular Plants a single spore- 
mother-cell, or connected group of them, or their products, this, together 
with its protective tissues, constitutes the essential of an individual 
sporangium.” In many cases the sporogenous group is not. strictly cir- 
cumscribed, but has ragged edges: cells which are sister-cells may not 
unfrequently be found to develop the one sterile, the other fertile. On 
the basis of structure this is consistent with the view that each fertile 
tract is a residuum left by advancing sterilisation. In the simpler stro- 
biloid types the sporangia are associated, singly or in small numbers, 
with appendages of various form and nature, which arise laterally, and 
in acropetal succession, as superficial outgrowths from the preé-existent 
axis: these are designated in various cases sporophylls or sporangiophores. 
The theory of the strobilus, stated in Chapter XI., uses the structural 
and developmental facts thus briefly summarised in the following way. 
It assumes, first, a sporophyte-body, already showing a distinction of a 
basal vegetative and an apical fertile region. This was endowed with 
apical growth, and an acropetal succession of its spore-development. The 
latter was relegated towards the surface, a change clearly indicated by the 
analogy of the Liverworts and Mosses. That by advancing sterilisation 
the fertile tissue underwent segregation into separate pockets, or sporangia, 
and that, by enation from the surface, appendages were formed in acro- 
petal succession, of the nature of sporangiophores, or sporophylls: upon 
these the fertile loculi would be borne outwards, as they are seen to be 
in the individual development of sporangiophores to-day. The apically 
growing axis would thus have been the pre-existent portion of the shoot, 
and the successively formed appendages secondary, as they are in the 
actual development. It has been shown that every one of these steps 
has its prototype among living plants: moreover the theory is in accor4- 
ance with the ontogeny at every step (Chapter XI). 
In the strobiloid type of the Lycopods the sporangia are definite in 
position and in number: while the relation of them to the bulky axis is 
very close. This is held to be a primitive condition, and palaeophy- 
tu ay shows that it was existent among the earliest fossils. In others, 
