INVERSION OF BALANCE 45 



protective wall of cells : it is dependent through life upon the gametophyte, 

 and results in a limited number of spores. In more complex Bryophytes 

 it is still short-lived and dependent, but larger, with distinction of apex 

 and base, a brief apical growth, and a basal vegetative region distinct 

 from the terminal capsule : there is entire absence of appendages, but 

 a partial differentiation of tissues, with internal ventilating system and 

 some assimilatory tissue. The spore-production is on a larger scale, but 

 limited usually to the simultaneous development of one continuous 

 spore-sac. 



In the Pteridophytes the mature sporophyte is an independent, self- 

 supporting organism ; but it is dependent in youth upon the parent 

 prothallus : it is commonly perennial. It has theoretically unlimited 

 growth of axis and root : the appendages vary greatly in size : there is 

 high differentiation of the tissues, with an elaborate ventilating system : 

 the plant thus constituted is capable of complete and continued self- 

 nutrition. The spores are produced after a more or less prolonged 

 vegetative phase, and in perennial forms their production may be 

 continued for an unlimited succession of seasons. They are borne 

 in separate sporangia, which are commonly seated upon the appendages : 

 the sporangia themselves are frequently produced in a continued succession. 

 These arrangements are such as to lead to a high and even long-continued 

 output of adequately nourished spores. The sporangia are frequently 

 restricted to certain shoots, in which the parts are closely aggregated : 

 these are termed strobili. 



The heterosporous state seen in all the highest Vascular Plants, 

 introduced advantages conducing to certainty in nursing the embryo, and 

 led in Seed-Plants to an infinity of special developments which secured 

 that transfer of the microspores which is so necessary for fertilisation. 

 But the essential plan of the independent, self-nourishing Vascular Plant 

 once laid down was not departed from, even in the highest forms. The 

 sporophyte, thus sprung from small beginnings, remains the dominant 

 generation in all distinctively terrestrial plants. 



The entire inversion of the balance of the two alternating generations 

 thus briefly sketched the dwindling away of the one and gradually 

 achieved dominance of' the other is a fact which requires some 

 physiological explanation. We may be sure that such things do not 

 happen without good reason. It will be our object later to enquire into 

 this. Meanwhile we recognise the fact itself, and we shall see in the 

 comparisons which lead to its recognition an enduring monument to 

 the genius of Hofmeister who first pointed them out. 



