THE EFFECT OF PARASITISM UPON THE PARASPrE— A STUDY IN 



PHYLOGENY. 



ERNST A. BESSEY. 



Amoug Zoologists it is generally recognizecl tliat parasitism and certain 

 types of morphological and anatomical changes go hand in hand. Thus among 

 the Copepods ayd Cirripeds the mature animals in some of the parasitic 

 species are scarcely more than sacks attached by some sort of sucking appar- 

 atus to the host and without any resemblance to the Crustaceans among whidi 

 they are classified. In these animals the relationship is determined with cer- 

 tainty because of the fact that the larval stages are typical for the group.s 

 concerned. Among the insects, the scales show considerable degeneration. 

 Many of the mites also show at maturity a much simplified condition. 



In most of the foregoing cases there exist numerous intermediate stages 

 between the free-living forms and the degenerate parasites. In the main, the 

 modifications that have occurred have been in the simplification of the body 

 structure of the mature individual. Thus extremities have been simplified or 

 lost, the alimentary canal reduced or obliterated, the sense organs lacking or 

 rudimentary ; only the reproductive organs are well developed, often with a 

 capacity for egg production far above that of their free-living relatives. 



Let us now turn to plant parasites. The first step toward parasitism 

 seems to be that of epiphytism, followed by partial or complete endophytism. 

 Apparently at first there is no physiological connection between the epiphyte 

 and its substratum. This step is found in nearly every group of chlorophyll- 

 containing plants. In the Myxophyceae it is represented by the one-celled 

 Chamaesiphon, which forms groups of gregarious cells on the filaments of 

 algae. They are paralleled in the Chlorophyceae by Characium and Trente- 

 pohlia, in the liryophyta by various Liverworts, and in the Anthophyta by 

 many epiphytic plants such as Tillandsia, Epidendrum, etc. Such epiphytes 

 are entirely holophytic and depend upon their hosts for nothing, or at least do 

 not draw upon the organic contents of any living host cell. Doubtless, how- 

 ever, many of the epiphytic orchids and bromeliads are dependent for their 

 mineral nutrients upon the decomposing bark to which their roots cling. 



The next step seems to have been partial or total endophytism. Among 

 the Chlorophyceae we find all grades of this, as well as in the Phaeophydeae, 

 Ilhodophyceae and other groups. Thus we have Chlorochytrium * living 

 between the cells of Lemna or other plants, apparently receiving little from 

 its host, but shelter aiid possibly such mineral salts as are present in the water 



♦See discussion (if tlic siippcisiMl p.irnsil ism of pliiiits of this kciiiis in Froi^inan, 

 K. M. Ol.scrviitioiis oil C'liloroilivl riiiiii. Minn. Bot. Studies "J :1!tr)-2')J, /'/. /y. l.S!ll». 

 lilst Midi, .\ciid. Sci. Ki-pt., 1!»1!». 



