FACTORS OF ADVANCE OF SPOROPHYTE 85 



extent, was intercalated. But spore-production follows sooner or later 

 in the normal life of every sporophyte, even of the most elaborate ; and 

 in the constancy of this process in all normal life-cycles is to be seen 

 one of the strongest supports of the antithetic theory. 



A second factor has been the segregation of sporogenous tissue, which 

 in the simpler forms is a concrete and continuous tract of cells, into 

 numerous more or less distinct pockets, or sporangia. Closely connected 

 with this, though not a necessary consequence of it, may follow the 

 increase or decrease in number of tlie sporangia thus distinct from one 

 another. There is good reason to believe that the methods of morpho- 

 logical advance in former times were not essentially different from those 

 to be seen reflected in the plants of the present day. If that be so, 

 then a careful study of the modes of variation of number of sporangia 

 seen to be effective now, should indicate the methods which have led 

 in the past to the condition seen in vascular plants, in which the number 

 of sporangia is as a rule large. 



A third factor has been the origin of the appendicular organs of the 

 shoot, and the origin of the roots. These changes have more than anything 

 else contributed to modification of the form of the plant-body. But though 

 these were such important steps, the mode of origin of the appendicular 

 organs and roots is still a subject for surmise rather than for definite 

 opinion. If, however, the development of the individual be accepted as a 

 guide, a reasonable view may be arrived at, which will be stated later. 



Beyond these, which are the three fundamental factors of advance of the 

 growing sporophyte, are others which, though later in point of time, have 

 had very far-reaching effects upon the organisms in which they appeared ; 

 such, for instance, as the abortion of parts : the initiation of the 

 heterosporous state, and of its ultimate consequence, the seed-habit : also 

 the adoption of the siphonogamic fertilisation, and of the various adaptive 

 methods for transfer of the pollen, as exemplified by the higher forms. 

 These factors are all closely related to the process of spore-production, 

 which we may regard in point of history to have been the primary function 

 of the sporophyte. 



On the other hand, the vegetative system, which we may regard as 

 being collectively secondary in its origin, has shown many characters 

 which may be held as adaptive. The differentiation of tissues has provided 

 first the means of construction of fresh organic material within the sporophyte 

 itself, so that in place of being a body dependent for its nutrition 

 upon the parent gametophyte, it became a physiologically independent, 

 self-nourishing organism. Further, the establishment of a conducting 

 system secured the necessary facility of transfer of materials from point 

 to point: this becomes specially necessary where the formation of appen- 

 dicuiar organs has brought about a large proportion of external surface 

 to bulk. Lastly, the appendicular organs themselves are open to 

 differentiation, so as to subserve definite functions : such as, absorption, 



