CHAPTER XVI 

 THE MEGASPORANGIUM AND SEED 



In the Pteridophyta the phenomenon of heterospory has de- 

 veloped in many different groups. The result of the realization 

 of this condition has been the appearance of smaller sporangia 

 producing numerous small spores known as microspores and of 

 larger ones giving rise to a few large spores designated as mega- 

 spores or macrospores. In the case of the sporangia which give 

 rise to megaspores, or the megasporangia, the conditions connected 

 with opening are the same as those exhibited by the mass of Pteri- 

 dophyta; in other words, the spore sacs are ectokinetic and owe 

 their dehiscence to the activity of a mechanical layer derived from 

 the epidermis. As this situation has been sufficiently discussed 

 in the previous chapter, it will not be profitable to return to the 

 matter here. Megasporangia in the proper sense of the word, 

 wherever they occur, are ectokinetic. 



It has been recognized since the times of the great German 

 morphologist Hofmeister that seeds represent modified mega- 

 sporangia. This view of the origin of seeds is justified, not only 

 by their anatomical structure and by the cytological conditions 

 observed in the development of the endosperm, but also by the 

 actual persistence of the megaspore membrane in the seeds of 

 many of the lower gymnosperms. If any remaining doubt existed 

 as to the origin of seeds from megasporangia, it would be removed 

 by the discovery of certain interesting structures in the case of 

 Paleozoic lycopods which present at the same time many of the 

 distinctive characteristics of megasporangia and seeds. So strik- 

 ingly does one of these (described by Scott) resemble a seed in its 

 external appearance that it was for a time actually regarded as one, 

 until its internal organization revealed its anomalous character. 

 Fig. 165 illustrates the vertical section of Lepidocarpon Lomaxi, 

 the seedlike fructification of a lepidodendrid. Internally is shown 

 a mass of cells, the gametophyte, surrounded by a heavy dark line, 



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