394 



LECTURE XXIV. 



commensalism with green Algae, assume forms otherwise proper only to typical chloro- 

 phyll-containing plants. It was certainly this totally different facies of the Lichens 

 as contrasted with other Fungi which chiefly contributed to make the new theory 

 of the nature of the Lichens so unacceptable to the older Lichenologists. 



Another highly important lesson may be drawn from the nature of the Lichens 

 as now understood, namely, that the external form of a plant is not at all de- 

 pendent upon its histological nature. Evidently it does not lie in the nature of the 

 fungus-hyphse that the fruticose and foliaceous Lichens assume such characteristic 

 forms as shrub-like bodies and flat extended surfaces, and still less do the Algae con- 

 cerned themselves — i.e. when living independently— tend to the formation of bodies 

 which assume the forms represented in Fig. 251, These forms however may well 

 result from the union of the two, and this chiefly according to the principle that in 

 the case of a plant containing chlorophyll it is important to present the green cells 

 in a suitable manner to the light and air. Finally, again to mention the fact, the 

 commensalism of Fungus and Alga brings about that the former no longer becomes 

 differentiated into a mycelium and fruit-bearing organs as is usually the case. In other 

 Fungi the mycelium is endowed with the properties of true roots, and penetrates into 

 the substratum in order to take up nourishment : the necessity for this disappears 

 in the Lichens. The Algae contained in the tissue constitute the nourishing substratum 

 for the hyphoe, and they must be exposed to the light and air, and thus, instead of a 

 mycelium, a body is developed which is attached to some solid support by organs of 

 attachment only at one or a few places, somewhat as in the case of many large Algae. 

 These organs of attachment in the foliaceous Lichens may assume the most essential 

 properties of roots. 



According to recent investigations, a commensalism similar to that between Alga 

 and Fungus in the Lichens appears to occur also between certain Algae and various 

 simply organised animals. It has long been known that some Rhizopods, Paramaecia, 

 Stentors, Vorticellas, the fresh-water Sponge {SpojigiUa), our small fresh-water Polyp 

 {Hydra), and various Rotifera ( Voriex) contain in the transparent substance of their 

 bodies green grains which have been supposed to be chlorophyll-corpuscles. Accord- 

 ing to a recent investigation of Brandt \ it appears now that these apparent chlorophyll- 

 corpuscles are small spheroidal algal cells provided with a nucleus, and further that 

 the animals concerned, when they contain such vegetable cells which are 

 capable of assimil.xtion, can dispense with the further absorption of food. These 

 animals are thus nourished like the Lichen-fungi by means of the Algae imprisoned 

 in them. If on the contrary they contain no Alg^, they are necessitated to feed as 

 animals in the ordinary manner — i. e. to take up nutritive substances through their 

 mouths. 



* Brandt, ' Über das Zmaminoikbcn von Thicrcii Jind Algcn,' Verhandl. der physiol. Ges. zu 

 Berlin (Dec. 2, 1881). 



