126 THE EVOLUTION OF PLANTS 



Besides the retention of the megaspore, there 

 is a third very general characteristic of the seed; 

 the megasporangium is enclosed in one or more 

 coats or envelopes; we already find this in the 

 still cryptogamic Water-fern Azolla. We will not 

 discuss the doubtful question of the nature of the 

 seed-coats; the most obvious comparison is with 

 the indusium of Ferns, a membranous envelope 

 which in some cases may enclose only a single 

 sporangium. At any rate, the coats are an essen- 

 tial part of the ovule or seed, which is thus not 

 simply a megasporangium, but a megasporangium 

 plus something else. In the young stage, or ovule, 

 the opening (micropyle) left by the seed-coats 

 receives the pollen in Gymnosperms, and forms 

 a passage for the pollen-tube in Angiosperms. 



We may now return to the seeds of Pterido- 

 sperms. They are true seeds, for they retain 

 the megaspore permanently within the nucellus, 

 which is the equivalent of the megasporangium, 

 and the nucellus is enclosed in one or more 

 coats. All the parts often have an elaborate 

 structure, quite equal to that of the Cycadean 

 seed, and there is little to show that these an- 

 cient seeds stand near the Cryptogamic sporan- 

 gia. It is true that in the seed of Lyginodendron 

 the neck of the pollen-chamber projected beyond 

 the seed-coats, and no doubt caught its own 

 pollen, without waiting for it to be brought in 



