LYCOPODIAI.KS 521 



The biological advantage which follows 011 the adoption of hetero* 

 spory lies in the fact that a large and well-nourished megaspore forms 



a better starting point for the new embryo than an independent 

 prothallns which has to elaborate its own supply. The megaspore 

 draws upon the ample resources of the parent sporophyte, and when 

 shed it already contains sufficient food to start the embryo well on 

 its way. This is seen in the germinating megaspores of Selaginella, 

 where the sporeling may attain considerable size before the store is 

 exhausted (Fig. 412, III.). Its root and shoot are then able to take 

 up nourishment independently of the prothallus. This heterosporous 

 condition had already been adopted by Palaeozoic Lycopods, so that 

 it is not a modern device. 



Adoption of the Seed-Habit. 



Certain of the Palaeozoic Lycopods had, however, gone a step 

 further than the state seen in Selaginella. They retained the megaspore 

 within the tissue of the parent, so that its nutrition could be continued 

 without the interruption caused by shedding of the spore. In Lepido- 

 carpon and Miadesmia a seed-like structure actually existed, though 

 not exactly of the same type as that which has become a constant and 

 permanent feature in Seed-Plants. A similar condition, but carried 

 out in greater perfection, is seen in certain Fern-like Plants of the 

 Palaeozoic Period. They are called Pteridosperms from the fact that 

 they produced seeds containing a megaspore. This retention of the 

 megaspore finds its biological justification in the fact that thereby 

 the nutrition of the germ is still more effectively secured than it is 

 by simple heterospory. // thus appears that certain Pteridophxta 

 illustrate steps essential in the institution of the Seed-Habit. These 

 are, first, the adoption of heterospory, and, secondly, the retention of the 

 megaspore upon the parent plant. 



The establishment of the Seed-Habit was undoubtedly the greatest 

 evolutionary advance towards a specific Flora of the Land. Its effect 

 is shown by the prevalence of Seed-Plants upon exposed Land- 

 Surfaces. In any ordinary landscape it is the Seed-Plants, whether 

 Angiosperms or Gymnosperms, which strike the eye, and appear to 

 make up the majority of the Vegetation. Though the Angiosperms 

 date back only to the Cretaceous period, the more primitive Gymno- 

 sperms are of much earlier origin, while the archaic Pteridosperms, 

 Fern-like Plants which bore seeds sometimes of considerable size, 

 carry the Seed-Habit far back into the Palaeozoic Age. Thus, 



