84 Mutations and Evolution. 



sporophyte has developed far enough to make possible the production 

 of spores in sufficiently large numbers to ensure the passage through 

 the difficult stage in the life-cycle when water is a necessity for the 

 accomplishment of fertilization. Only in vascular plants above the 

 Cycads has complete adaptation to terrestrial conditions been 

 accomplished, by the adoption of siphonogamy and the loss of 

 swimming sperms. 1 Nevertheless, the higher siphonogamous plants 

 do not, except in rare instances, show recapitulation stages of their 

 gametophytes. In other words, the gametophyte development is 

 usually direct. This can hardly be because the gametophyte in its 

 later evolution forms a reduction series, because among animals the 

 most remarkable and convincing instances of recapitulation occur 

 where degeneration has taken place as a result of parasitism or the 

 adoption of sedentary habits. Typical recapitulation phenomena 

 do nevertheless appear in plant gametophytes. For instance the 

 archegonium, which is so characteristic an organ in Bryophytes, 

 and has given an aggregate name to the three great groups in which 

 it occurs, persists in a progressively reduced form throughout the 

 Gymnosperms until we reach the higher Gnetales, although in certain 

 Araucarians (Barnes, 1913) it has become a positive hindrance to 

 fertilization. In this case the archegonium neck cells have developed 

 into a thick-walled structure which the pollen tube cannot penetrate. 

 But the jacket-cells adjacent to the neck are actually eliminated, 

 thus making it easier for the pollen tube to reach the central cell. 

 No more striking case could be cited of the continued production of 

 an organ which not only has lost its function but which is a positive 

 hindrance to the functioning pollen tube, though its retention has 

 entailed other structural changes in the jacket-cells to facilitate 

 fertilization. 



Thus throughout the Gymnosperms, the gametophyte is being 

 reduced, and recapitulation phenomena which suggest an ebbing 

 tide occur in its terminal stages. The archegonium itself shows a 

 gradual series of reduction stages until it is finally eliminated. In 

 the lower Gymnosperms it has already lost the neck canal cells 

 found in the archegonium of Mosses and Perns, and among the 

 Conifers the ventral canal cell is gradually eliminated. In the 

 Abietinea (see Coulter and Chamberlain, 1910) a ventral canal cell 

 is cut off, in the Araucarians the nuclear division takes place but 

 no cell wall is formed ; or in some species of Pinus a wall may be 



1 It is well known that similar types of adaptation from aquatic to 

 terrestrial conditions have occurred in the Fungi, involving the loss of free 

 swimming gametes and the development of some form of siphonogamy. 



