VII PTERIDOPHYTA—FILICJNEJE—OPIJJOCI.OSSACRAi 231 



aquatic organism like the gametophyte of all Muscineae. In the 

 few cases where true roots are absent their phce is taken by 

 other structures that perform their functions. The assimilative 

 activity is restricted to special organs, the leaves, except in a few 

 cases where these become much reduced, as in Psilotum or Eqiii- 

 sctmn. A main axis is present upon which the leaves are borne 

 as appendages, and this continues to form new leaves and 

 roots as long as the sporophyte li^/es. 



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 liryo- 

 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 typical Ferns there 

 are four of these primary growing points, giving rise respect- 

 ively to the stem, leaf, root and foot. The latter is a tem- 

 porary structure, by which the young sporophyte absorbs food 

 from the gametophyte, but as soon as it becomes independent 

 the foot 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 characteris- 

 tic of these is 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 Anthocerotes ; 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 game- 

 tophyte, from the large long-lived prothallium of the Marat- 

 tiacere 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 Hepaticse, and especially with the 

 Anthocerotes, shows that the difference is not so great as it at 

 first sight appears. A further discussion of this point must be 

 left, however, until we have considered more in detail the struc- 

 ture of these parts in the different groups of the Pteridophytes, 



