1 68 Practical Plant Biology. 



Notwithstanding the much more complex structure of the sporo- 

 phyte of Aspidium and the great differences between the forms of 

 the gametophytes of the ferns, the mosses and the liverworts, the 

 life-histories of these are evidently similar and the structure of their 

 sexual organs resemble each other in many respects. These broad 

 resemblances have led botanists, in the first instance merely for the 

 sake of convenience, to classify these three sorts of plants together 

 under the name of Archegoniates, all being characterised by the 

 possession of a multicellular archegonium which contains the ovum. 

 These same characteristics distinguish them from the more simply- 

 constructed Algae and Fungi which, almost without exception, 

 have their ovum retained in an oogonium of unicellular construction. 

 In contradistinction to the Archegoniates the Algae a'nd Fungi are 

 described as Thallophytes, a name suggested by the fact that their 

 plant body is never differentiated into stem, root and leaves. 



When we compare the different kinds of plants included in a 

 great group like Archegoniates, in addition to the broad resem- 

 blances just noticed, a correspondence of the parts in these 

 different plants is forced upon us. For example, the correspond- 

 ence of the gametophyte of Marchantia, arising from a spore and 

 culminating in the production of sexual cells, with that of Aspidium 

 is evident, and the correspondence of the sporophyte of Funaria 

 with that of Marchantia is equally clear. Again, the theca of 

 Funaria corresponds with the capsule of Marchantia and the 

 archegonium of Aspidium corresponds with the archegonium 

 of Funaria. Such parts of different plants which correspond 

 to one another in their origin, in their relation to the other 

 parts of the plants considered and in their position in the life- 

 history, are termed homologues, and such parts are said to be 

 homologous to one another. Homologous parts may, and often 

 do, discharge the same function, but this is by no means always 

 the case ; thus the sporophyte of Marchantia does not carry on 

 photosynthesis, while that of Funaria does to a slight extent, and 

 the sporophyte of Aspidium is ' very efficient in this respect. 

 This difference in function does not, however, prevent these three 

 parts from being homologous. Again, similarity of function does 

 not necessarily indicate homology. This will be readily under- 

 stood from the comparison of the leaves of the moss, Funaria, with 

 those of Aspidium. They both exercise the function of photo- 

 synthesis ; but their positions in the life-history are wholly different. 

 The leaves of Funaria are organs of the gametophyte while those 

 of Aspidium belong to the sporophyte. They are therefore not 

 homologous. Such parts performing the same function are said to 



