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 



