1448 LIFE : OUTLINES OF GENERAL BIOLOGY 



together in mutually beneficial partnership or symbiosis. The 

 fungus absorbs water and salts, the alga accomplishes photo- 

 synthesis, and lichens will flourish where no other kind of plant 

 can survive. Later came continuators, notably Bornet, Stahl, and 

 Rees, who demonstrated not only strange unions with fresh fullness 

 and clearness, but were able, for instance, to build up a lichen by 

 combining the appropriate alga and fungus. We give prominence 

 to this elucidation of the true inwardness of lichens, because it l| 

 prepared the biological mind for the discovery of the frequent 

 occurrence of symbiosis. It is widely, though not unanimous!}-, held 

 among botanists that the heather flourishes because it is interpene- 

 trated by a partner fungus; many orchids refuse to grow and their 

 seeds to germinate without their "mycorhiza" ; and such fungal 

 symbiosis is also characteristically presented in the roots of many 

 trees. Leguminous plants have their well grown and often nutritive 

 foliage, their exuberant flowering, and their richly protein-contain- 

 ing seeds explained by help of symbiotic bacterial allies in their 

 root-tubercles, which assimilate nitrogen from the atmosphere ; and 

 there are not a few other instances of plant partnering plant. All 

 Radiolarians bear small "yellow cells" which turn out to be 

 symbiotic Algae ; and similar partners to these occur in the animal 

 world here and there, as in freshwater sponges and green hydra, in 

 various sea-anemones and corals, and in the green Convoluta, a 

 simple marine planarian worm, of sandy shores in Brittany, lately 

 well re-investigated by Keeble. 



The life of many insects, such as beetle-larvae that feed wholly 

 on dry wood, becomes, to say the least, more intelligible since the 

 fermentative aid of their intestinal yeast-plants and bacteria has 

 been recognised. So plant partners animal. All wood-eating Termites 

 have in their food-canal large numbers of Infusorians, not found 

 anywhere else, without which they cannot utilise their dry-as-dust 

 food. So animal partners animal. 



Another biological foundation-stone, which many helped to lay, 

 was the demonstration that all growing and development is asso- 

 ciated with the process of cell-division, and that in the ordinary 

 division process the chromatin of the nucleus is divided with meti- 

 culously exact equality between the two daughter-cells. Whereas 

 the cell-substance or cytoplasm undergoes what may be called a 

 mass-division, each of the chromatin bodies or chromosomes is 

 split longitudinally up the middle, a half going to each daughter- 

 cell. As this occurs with constancy in the usual process of cell- 

 division throughout the world of life, beginning even in some of the 

 unicellular organisms, it must have a deep significance, part of 

 which is that the chromosomes are the vehicles of many, if not all, 

 of the hereditary and specific characters. 



The forces at work in such normal (mitotic or karyokinetic) cell- 



