ORIGIN OF STERILE REGION SECONDARY 161 



present day will justify the assumption that in the evolutionary story 

 sporangia originated indiscriminately upon pre-existent vegetative organs. 

 My own opinion is that it does not, for a careful examination of such 

 cases and comparison of them with the general type to which the plants in 

 question belong shows that they are exceptional, if not indeed of the nature of 

 monstrosities. It is clear that promiscuous formation of sporangia in 

 present-day forms is possible, and that it does at times occur, but it 

 does not follow that this was a general mode of their origin in past 

 times. 



An essential fact bearing upon the question in point is that spore- 

 production is a constantly recurring event in Archegoniate Plants. There 

 is good reason to believe that it has found its place in every normally 

 completed life-cycle throughout their descent. Cytologically it is now 

 seen to be the natural complement of the sexual process. Taking all 

 types of Archegoniate Plants into our view, including the more recent 

 Flowering Plants as well, there is reason to believe that spore-production 

 was the initial function of the sporophyte, and that it has been continued 

 and repeated throughout descent. If this be admitted, how can the 

 strobilus, or the flower the part bound up with that primary function 

 of spore-production be the result of metamorphosis of a vegetative shoot, 

 the leading function of which is secondary? The conclusion to be derived 

 from broad comparison will be the direct converse: viz. that vegetative 

 parts in the sporophyte have originated by change of parts originally 

 fertile. 



But in order to carry conviction that this conclusion is correct, it will 

 be incumbent on those who hold it to bring forward evidence bearing on the 

 origin or increase of the vegetative system, which we see at the present 

 day preceding spore-production in the history of the individual life. It 

 has already been shown in Chapter VII. that sterilisation of individual 

 sporogenous cells, that is, their conversion into cells having a vegetative 

 function, is common. It is found in the sporangia of Vascular Plants, 

 but it is in the sporogonia of Bryophytes that it has been recognised as 

 specially effective in adding to the vegetative system. The sporogonium 

 of Aneura (Fig. 86) has already served as an example, while reference to the 

 writings of Goebel (Organography, pp. 326-329, Engl. Ed., p. 103), shows 

 how fully sterilisation has already been realised, and accepted as a source 

 of increase of the vegetative system in the Bryophyta. Similarly, in 

 Vascular Plants it has been shown above, that sterile cells of a sporo- 

 genous group may be converted into vegetative tissue of a septum. Such 

 examples indicate how sterilisation of individual cells may be effective in 

 increasing, and perhaps in the first instance even in originating, the vege- 

 tative system. 



A second factor, which has been specially effective in contributing to 

 the increase of the vegetative system in the more differentiated types of 

 sporophyte, is the abortion of sporangia, or of sporangium-bearing parts. 



