12 111 I' JOUBNAL OF IJOTANV 



these arc inside the calcified test, and Cryptoinonads are not known 

 to be doininantly holophytic. 



On the.other hand, one of the most perfect examples of animal 

 decadence and dependence on the intruder is afforded by the ease of 

 the green worm Convoluta roscoffensis \ in which complete decadence 

 of the metabolic organization of the host follows readily obtained 

 food-supply by the photosynthesis of helot algye of the type of 

 Carteria of the Ohlamydomonads 2 . Infection takes place in a larval 

 stage, via the mouth, and is obligatory. With the alga growing and 

 dividing, the host ceases to feed, but in absence of the algae it 

 dwindles out; on the other hand, the algal units are reduced to 

 complete helotism as residual chloroplasts. 



In all animal-examples of such commensalism, symbiosis, or 

 dependence, it may be noted once more that there is never any 

 question of 'dual control,' and that there is never the slightest indi- 

 cation of a special somatic factor being introduced as implying 

 a new departure in the somatic organization as a ' consortium. ' The 

 expression 'intrusion' covers all the cases. Where there is no 

 mechanical hindrance, anything may invade anything, in the chances 

 of a moving medium ; such intrusion being but an extended phase of 

 older processes of 'nutrition' by ingestion at the surface of freely- 

 exposed cytoplasm. Suggestions of special adaptations in more success- 

 ful plant and animal phyla to keep out such intrusions have been 

 noted, as in the case of the abundant screen -formation of mucilage 

 hairs in many seaweeds: the ' trichocysts ' of flagellates {JBouchetia, 

 Polylcrilcos) have been freely interpreted from this standpoint. The 

 general organization of higher animals with protective exoskeleton, 

 and internal alimentary canal still exposed to ' infection ' at its 

 digestive surface, follows the same principles; and even in the plank- 

 ton-phase the first inception of the cellulose or chitinous membrane 

 has been regarded as owing its persistence in phylogeny to its value 

 as a secondary utilization of the debris-heap of waste polysaccharides 

 to this end. 



Admitting the general facts and wide distribution of such pheno- 

 mena of intrusion in early marine organism, botanical- interest next 

 centres in the manner in which such intrusive photosynthetic units 

 may be utilized as a source of carbohydrate by the more dominant 

 heterotrophic fungus of the Lichen-association ; since, though the 

 algal cells may be intrusive in the fungus-soma, they are not as in the 

 preceding animal-forms intrusive in the actual plasma of the host. 

 But even here a broader view is required from the analogy of plant- 

 life as known on the land. Heterotrophy is a phenomenon of signi- 

 ficant importance in all massive plant-growths, even in the sea to the 

 special factors of which it owes its inception ; in that all tissues 

 beyond the range of penetration of light must necessarily live at the 

 expense of the surface -layers — over a range, that is to say, of rarely 

 more than 100 /a. Light-penetration in the case of subaerial vege- 

 tation may be much more effective ; but 'the general fact that hetero- 



1 Keeble and Gamble (1907), Q. J. M. S. li. 2. Koeble, Plant-Animals (1910), 

 p. 123. 



2 Doflein (191G), Protozoenkunde, p. 447. 



