CH. VIII THE PTERIDOPHYTA—OPHIOGLOSSACEyE 219 



where these become much reduced, as in Psilotum or Equisetum. 

 A main axis is present upon which the leaves are borne as 

 appendages, and which continues to form new leaves and roots 

 as long as the sporophyte lives. 



The differentiation of these special organs begins while 

 the sporophyte is still very young. The earliest divisions in the 

 embryo correspond closely to those in the embryo of a Bryo- 

 phyte, but instead of forming simply a capsule, as in all the 

 Bryophytes, there is established more than one growing point, 

 each one forming a distinct organ. In the Ferns there are 

 four of these primary growing points, giving rise respectively 

 to the stem, leaf, root, and foot. The latter is a temporary 

 structure, by which the young sporophyte absorbs food from the 

 gametophyte, but as soon as it becomes independent this 

 gradually withers away, and soon all trace of it is lost. 



The originally homogeneous tissues of the embryo become 

 differentiated into the extremely complicated and varied tissues 

 characterising the mature sporophyte. The most characteristic 

 of these are the vascular system of tissues. This is hinted at 

 in the central strand of tissue in the seta of many Mosses, and 

 the columella of the Anthoceroteae ; but in no Bryophyte does 

 it reach the perfect development found in the Ferns and their 

 relations, which are often called on this account the vascular 

 cryptogams. 



The gradual reduction in the vegetative parts of the 

 gametophyte, from the large long-lived prothallium of the 

 Marattiaceae to the excessively reduced one found in the 

 heterosporous Pteridophytes, has already been referred to in 

 the introductory chapter. 



The structure of the sexual organs of the Pteridophytes 

 appears at first sight radically different from that of the 

 Bryophytes, but a careful comparison of the lower forms of the 

 former with some of the Hepaticae, especially the Anthoceroteae, 

 shows that the difference is not so great as it at first sight 

 appears. A further discussion of this point must be left, how- 

 ever, until we have considered more in detail the structure of 

 these parts in the different groups of the Pteridophytes, where 

 they are remarkably uniform. In all of them the archegonium 

 has a neck composed of but four rows of peripheral cells, instead 

 of five or six, as in the Bryophytes, and the antheridium, except 

 in the leptosporangiate Ferns, is more or less completely sunk 



