276 PHYSIOLOGICAL DEVELOPMENT. 



organization is so low neither part can reproduce the other 

 and survive by so doing, is probably due to the circumstance 

 that neither part contains any considerable stock of untrans- 

 formed protoplasm, out of which new tissues may be pro- 

 duced. 



Fungi and Lichens present no very significant advances 

 of integration. We will therefore pass at once to the 

 Acrogens. In those of them which, either as single fronds 

 or strings of fronds, spread over surfaces, and which, rooting 

 themselves as they spread, do not need that each part should 

 receive aid from remote parts, there is no developed vascular 

 system serving to facilitate transfer of nutriment : the parts 

 being little differentiated there is but little integration. Jiut 

 along with assumption of the upright attitude and the ac- 

 companying specializations, producing vessels for distribu- 

 ting sap and hard tissue for giving mechanical support, there 

 arises a decided physiological division of labour ; rendering 

 the aerial part dependent on the imbedded part and the im- 

 bedded part dependent on the aerial part. Here, indeed, as 

 elsewhere, these concomitant changes are but two aspects of 

 the same change. Always the gain of power to discharge a 

 special function involves a loss of power to perform other 

 functions ; and always, therefore, increased mutual dependence 

 constituting physiological integration, must keep pace with 

 that increased fitting of particular parts to particular duties 

 which constitutes physiological differentiation. 



Making a great advance among the Acrogens, this physio- 

 logical integration reaches its climax among Endogens and 

 Exogens. In them we see interdependence throughout 

 masses that are immense. Along with specialized appli- 

 ances for support and transfer, we find an exchange of aid at 

 great distances. We see roots giving the vast aerial growth 

 a hold tenacious enough to withstand violent winds, and 

 supplying water enough even during periods of drought ; we 

 oee a stem and branches of corresponding strength for up- 

 holding the assimilating organs under ordinary and extraor- 



