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 VIII.), 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 pre-existent 

 axis : these are designated in various cases sporophylls or sporangiophores. 

 The theory of the strobilus, stated in Chapter XL, 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 accord- 

 ance with the ontogeny at every step (Chapter XL). 



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- 

 tology shows that it was existent among the earliest fossils. In others, 



