\ 4 o GENERAL BIOLOGY 



The macrospore likewise begins its development while in 

 the sporangium. It contains, however, within its own wall a 

 store of food material, upon which considerable develop- 

 ment is possible. The spore divides repeatedly, and grow- 

 ing at the expense of the stored food, bursts the spore wall, 

 and protrudes as a small prothallium which develops a few 

 rudimentary rhizoids, and later a few archegonia containing 

 egg cells. These are developed and often fertilized before 

 the growth of the prothallium is complete. Since the 

 microspores are developed in the upper part of the fruiting 

 cones they may fall down into the lower scales. 



The fertilized egg develops foot, stem, root and leaf as 

 before, and in addition a special embryonic organ the so- 

 called suspensor, whose function it is to keep the embryo 

 pushed down against the prothallial tissue from which it 

 must obtain its food. By the time this supply is exhausted 

 the embryo has developed a root, bearing rhizoids, and one 

 or two pairs of minute leaves at the apex of the stem, and is 

 ready to get food for itself independently. 



The predominance of the sporophyte phase is in this 

 plant, very marked. The gametophyte is here not only 

 ^reduced in size, but wholly dependent on the antecedent 

 sporophyte for food the reversal of the conditions with 

 .which we started. 



Study 18. Fern development. 



Materials needed: Fronds, bearing ripe and immature 

 sporangia ; prothallia in all stages of development and old 

 ones bearing sporophytes. 



Study the grouping of the sporangia in relation to each 

 other and to the veins of the leaf, and their protective 

 covering. 



Study the structure of the mature and of the ruptured 

 sporangia. 



