90 ON THE STRUCTURE AND FUNCTIONS OP FERNS. 



scattered about in the form of fine dust. In Osmunda, 

 JBotrychium, and Ophioglossivm, the spore-cases have 

 not the elastic ring, but are composed of two valves. 



In the greater part of the British species, the sori 

 are covered during the earlier period of their growth 

 with a thin membranous covering, which is called an 

 indusium. Some ferns possess this organ, which is 

 very evident when they are young. It is, however, 

 cast off as the sori attain maturity. The presence 

 or absence of this indusium is looked upon as an im- 

 portant point in the economy of ferns, and they are 

 divided according to this fact. 



Although the spore is often called the representa- 

 tive of the seed, this little structureless body truly 

 represents the bud in higher plants. If we watch 

 the development of the spore, we shall find that it 

 does not directly grow up into a fern, but that, after 

 having attached itself to a damp piece of earth or rock, 

 it begins to expand into the form of a green mem- 

 brane. This green membranous mass, on account of 

 its resemblance to the fronds of the common Mar- 

 cJiantia, has been called the Marchantia-like expansion 

 of the fern. It is also called the prothallus. On the 

 surface of this little body, there appear in the course 

 of time two sets of organs. The one called pistttlidla, 

 containing in their interior little ovoid bodies, which 

 are the representatives of the ovules or seed-buds of 

 the higher plants. The other organs are called arche- 

 gonia : they contain little movable worm- like bodies, 

 called spermatazoids, and which are found in many of 

 the lower forms of plants. These bodies represent 

 the pollen or pollen-tube of the flowering plants. 

 They find their way to the ovules contained in the 

 pistillidia ; and it is after the mixture of the produce 

 of these two different cells that the young fern shoots 

 forth from the surface of the prothallus, and grows up 

 into a new plant. 



