Aridity and Evolution. 



221 



in other features of the plant, evolutionary development was slow 

 so long as the monotonous condition of an aquatic habitat were 

 to be met. 



The present occasion does not permit a discussion of the 

 probable evolutionary development of the vegetal organism 

 from the stage of simple colloidal masses to the gametophyte, 

 and the evidence at our disposal is so lacking that such discussion 

 would be entirely speculative. It will be quite pertinent to call 

 attention to the fact, however, that these earlier rhizoidal anchor- 

 age organs stand in no direct or genetic relationship to the roots 

 of modern seed-plants. 



THE ORIGIN OF A LAND FLORA. 



As has been so succinctly described by Bower, types of 

 vegetation with gametophytic reproduction must necessarily 

 remain aquatic, or at least hygrophilous, since free water was 

 necessary for the movements of gametes in effecting fertilization. 

 Furthermore, the stimulus of desiccation, when such forms were 

 stranded or left above the water level, does not appear to 

 have any direct consequence in the way of development of inci- 

 dental structures which might facilitate such sexual reproduc- 

 tion, a fact probably due to the morpho genetic limitations of 

 gametophyte, not easily understood. 



The problem of aridity was in reality solved in quite another 

 way by the prolongation of the vegetative existence of the ger- 

 minating zygospores, formed by the union of the gametes, 

 finally resulting in the independent sporophyte. The sporo- 

 phyte was not dependent upon free water for any part of its 

 existence and its individual occupation of drier areas was ac- 

 companied by a development of the anchorage organ anew, 

 this time from a highly developed shoot-like axis, and the dif- 

 ferentiating effects of dessication upon the root have been scarcely 

 less marked than those affecting the shoot. 



The necessity for anchorage was even greater than before, 

 but now the nutritive salts no longer bathed the entire body but 

 were present only in hygroscopic layers on the soil particles with 

 a vertical distribution not uniform and with much horizontal 

 irregularity. The formation of absorbing mechanisms and con- 

 ducting tissues has been followed by a refinement cf fcrm and 

 habit reaction in the modern plant, as shown by the researches 



