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 the 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 he 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- 
dicular 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, 
