HETEROSPORY AND THE SEED-HABIT 707 
A second consequence of the adoption of the Seed-Habit was the 
continued nutrition of the embryo by the parent plant: not only was 
accurate fertilisation secured, but the embryo was far advanced in its 
development, and supplied with a large nutritive store before being isolated, 
and becoming dependent on its own resources. This, together with the 
mechanical protection of the seed-coat, brings a highly increased certainty 
of establishment of each germ as a new individual. Economy will again 
follow on the increased chance of success of each individual germ, and 
the general tendency of these precise and certain arrangements must 
have been in the direction of reduction: evidence of this is to be recognised 
generally in the floral construction of Seed-Plants. Amid all the fluctuations 
of detail of the floral mechanisms they show, as compared with the 
Pteridosperms or Cycadales, evident traces of that reduction which the 
adoption of the Seed-Habit would on biological grounds lead us to expect. 
The higher terms of the series of Vascular Plants show more exact 
differentiation of the vegetative and reproductive systems than the lower. 
Each appears to have taken independently its own line of specialisation. 
But there is good reason to hold these advances as mere changes of detail 
in a plan substantially the same, however important may be the biological 
effects thus gained. The general plan of the shoot of Flowering Plants, 
whether vegetative or propagative, and the characters of its several parts 
remain the same as in the more primitive Vascular Plants, though subject 
to an infinity of modifications ; and the conclusion which is forced upon the 
mind in contemplating the construction of Vascular Plants at large is, the 
unity of the general scheme underlying them all. It is based, as we have 
seen, on the individual shoot, consisting of an apically-growing axis with 
appendages borne in acropetal succession, and accessory roots. The 
general-purposes shoot, as seen in its essentials in the earliest homosporous 
Pteridophytes, is the pattern: from this, by segregation of the vegetative 
and propagative regions, and subsequently by their independent specialisation, 
even the highest terms of the Flora of the Land may be held to have been 
derived. And in the course of this evolution there is evidence of two 
“main progressions as regards the size of the appendages, and their pro- 
pagative capacity. In the first and more primitive phase, which was 
characterised by being homosporous, there are comparative reasons which 
have been explained at length above for recognising a very general ampli- 
fication, though subject in special cases to reduction. This is in accordance 
with the obvious biological advantage in homosporous forms of producing 
as large a spore-output as possible. It involved in some cases profuse 
branching of the shoot, while the individual appendages remained small, 
as in the microphyllous Lycopodiales. In other cases the axis was not 
greatly, extended, nor the appendages numerous, but the latter made up for 
these deficiencies by their extensive individual growth and ramification. 
This is exemplified in the megaphyllous Ophioglossales and Filicales, 
while the sporangiophoric Pteridophytes take an intermediate place. Thus 
