Januaky 16, 19U3.] 



SCIENCE. 



103 



tative tissue as well, and finally attains 

 the status of an independent plant. 



The highly organized sporophyte of the 

 higher arehegoniates is connected with the 

 lower types by an almost continuous series 

 of existing forms, and through these with 

 the still simpler structures found in the 

 green algse. The increased output of 

 spores, with a corresponding number, of 

 new plants resulting from a single fer- 

 tilization, is an obvious advantage, and 

 undoubtedly is the explanation of the 

 origin of the sporophyte. 



If we compare the sporophyte of even 

 the simplest liverwort with that of the 

 algse, there is noted an essential difference. 

 The spores, instead of being motile zoo- 

 spores, are non-motile, thick-walled struc- 

 tures, adapted to resist drying up— in 

 short, the sporophyte • is a structure essen- 

 tially fitted for an aerial existence. Ex- 

 cept in the very lowest types, there is de- 

 veloped a special massive absorbent organ, 

 the foot, which is not unlike the root de- 

 veloped in the higher types, and is very 

 different from the delicate rhizoids of the 

 gametophyte. The latter always shows, to 

 a greater or less degree, its aquatic origin. 



From the time that the sporophyte has 

 attained the dignity of an independent ex- 

 istence, its development proceeded on lines 

 very different from those followed by the 

 essentially aquatic gametophyte. As we 

 have seen, the efforts of the latter to as- 

 sume a terrestrial habit have met with only 

 partial success, and it would appear that 

 nature concluded to try again, taking as a 

 starting point the essentially terrestrial 

 sporophyte, which, as a functionally new 

 development, seems to have proved more 

 plastic than the gametophyte. 



From the first, and this I believe to be 

 highly significant, its water supply was 

 obtained indirectly through the medium of 

 a special organ, the foot. It is not impor- 

 tant for a consideration of the question 



whether the foot ia all forms is or is not 

 homologous— enough that we find for the 

 first time an organ sufficiently massive to 

 supply all the water needed by the tissues 

 of the developing sporophyte. The foot 

 is a very different organ from the delicate 

 rhizoids of the gametophyte, and much 

 more like the true roots of the vascular 

 plants, which, it is highly probable, arose 

 as further modifications of the foot of the 

 sporogonium of some bryophyte. 



With the massive root penetrating the 

 earth and thus establishing communica- 

 tions with the water supply, the sporophyte 

 becomes entirely independent. The pos- 

 session of an apical meristem in the root 

 allows of unlimited growth, and gradually 

 the massive root system of the higher 

 plants has been evolved, keeping pace with 

 the increase in size of the Sporophyte, 

 which, except with rare exceptions, obtains 

 its whole water supply through the roots. 

 Correlated with this increase in size of the 

 sporophyte has been developed the char- 

 acteristic conducting tissues which consti- 

 tute the vascular bundles. While rudi- 

 mentary vascular bundles are found in the 

 sporophyte of many mosses and in Antho- 

 ceros, the characteristic tracheary tissue, 

 par excellence the water-conducting tissue 

 of the vascular plants, occurs only among 

 the latter forms. 



With the establishment of the sporo- 

 phyte as an independent plant, the gameto- 

 phyte serves mainly to develop the sexual 

 reproductive organs from which the sporo- 

 phyte arises. While the gametophyte 

 among the lower pteridophytes is a rela- 

 tively large and independent green plant, 

 sometimes living for several years, it be- 

 comes much reduced in size among the 

 more specialized heterosporous types, and 

 may live but a few hours, as in species of 

 MarsHia. In such forms little or no 

 chlorophyll is developed by the gameto- 

 phyte, which depends for its growth upon 



