SAPROPHYTISM AND SYMBIOSIS 803 



(p. 659), but some forms of temperate regions ( Usnea, for example) 

 may be partially parasitic on trees. A remarkable tropical lichen is 

 Cora Pavonia, in which the fungal symbiont, one of the Thelephoraceae, 

 may live entirely apart from algae, or symbiotically either with the alga, 

 Chroococcus, or with the alga, Scytonema, the body form differing in each 

 case. The shape of this lichen varies also with the proportional develop- 

 ment of the symbionts, the so-called genus Dictyonema representing 

 a bracket-like form in which the fungal element dominates, while the 

 so-called genus Laudatea represents a felt of filaments in which the alga 

 (i.e. Scytonema) dominates. Botrydina vulgaris, which commonly is 

 regarded as an alga, is invested by fungal hyphae, and is thought by 

 some investigators to be a primitive lichen. A lichen such as Cora sug- 

 gests that the fungal symbionts of other lichens may once have had 

 facultative relations with algae, and also that the body form is very 

 likely to have resulted from the symbiosis. 



Green-celled animals. Among the most remarkable of organisms are certain 

 green-celled animals (such as Spongilla, Hydra, and Convoluta), which in some 

 important respects resemble lichens. It has long been believed that the green cells 

 represent enslaved algae, though, in contrast with the algal symbionts of lichens, 

 separate cultivation generally is impossible. Indeed, the resemblance to algae is 

 much less than in lichens, consisting of little more than the presence of chlorophyll ; 

 usually even the nuclei are absent from the cells, and the chlorophyll may be dis- 

 seminated through the cell sap instead of being in plastids. Much light has been 

 thrown on these strange organisms by a careful study of Convoluta roscoffensis, one 

 of the flat-worms. This animal is colorless when hatched, but in the first few days 

 it becomes infected by motile algae (appearing to belong near Carteria), which seem 

 to exhibit prochemotactic reactions to substances in the egg capsules. At first the 

 animal has a mouth and feeds like other flat-worms, but soon the opening becomes 

 closed and the worm henceforth is completely dependent upon its symbiotic alga; 

 even excretory organs are wanting in the adult, it being supposed that the algae 

 utilize the excreta as a source of nitrogenous food. If the appropriate alga is 

 absent in a culture of the worms, the animals soon die, even in the continued presence 

 of such food as they previously have used. After imprisonment the algae lose their 

 motility, though active cell division takes place for some time. The new cells have 

 no cell walls, and eventually they become distorted ; finally the nuclei disappear and 

 all activity ceases. Such modified algae are unable to live apart from the worm, 

 and the worm cannot live apart from the algae; indeed, the algae of adult worms 

 cannot infect young worms, so that when they die, they leave no progeny. All 

 doubt as to the reality of symbiosis has been removed by synthesizing the composite 

 organism from pure cultures of the alga and the worm, precisely as lichens previously 

 were synthesized from cultures of algae and fungi. While the exact nutritive inter- 

 relations are not certainly known, it has been shown that the mature animal with its 

 green cells is prophototropic, and that starch is manufactured and oxygen given off 



