No. 506] KELPS AND RECAPITULATION THEORY 9r> 



would become: "The developmental stages of an organ- 

 ism are only the physiologically necessary steps for the 

 formation of its adult body from its earliest stage, which 

 is in most cases the egg." This is definite and it can he 

 readily tested by the facts, while the other is so vague as 

 to be scarcely susceptible of any such test. There is no 

 middle ground between these two alternative interpreta- 

 tions of the statement. For if an organism is found to 

 which it will not apply if somewhat but not greatly sep- 

 arated stages be considered, all that is 'necessary is to 

 take shorter and shorter stages until finally any ontogeny 

 must conform to it. 



Let us apply then, His's view, thus interpreted, to the 

 kelps. We have so far confined the account to the ex- 

 ternal morphology and have said little about their histol- 

 ogy. This will be of interest here. The general plan of 

 structure is the same in both stipe and lamina and similar 

 in all kelps. Within the epidermis is the cortex composed 

 of polygonal or rounded cells which may be thickened and 

 hardened to form strengthening tissue. Within this is a 

 pithweb made up of irregularly interlacing filaments 

 which sometimes show very remarkable differentiation. 

 Oliver ('87) first worked out in detail, showing that in 

 Macrocystis and Xereocystis, especially, sieve tubes are 

 developed which form a regular zone of vertiea %e *^ s ^ 

 around the less differentiated center of the pith. M<" 

 sieve plates of these become obliterated l»> the loimatM n 

 of callus as in the spermatophytes. r I here is goo< i < . 

 to believe that they are efficient in the transter ot mate- 

 rials from one part of the plant to another and tll<M1 J^~ 

 session may have made it possible for these plant- to at- 

 tain the great lengths they sometimes reach. The simp ei 

 internal pith consists of interlacing branching l.>p '^ 

 which run in all directions. Many of these meet aiu ^ 

 their junctions develop sieve plates connecting t iem ^ 

 one another, at the same time becoming swollen at t ie < n^ 

 like the flare of a trumpet. Such trumpet hypn« are ^ 

 mon in most members of the Lammanaceae. 



