ANGIOSPERMS 331 



algae, or chlorophyll-bearing thallophytes, and the fungi, or 

 those which do not bear chlorophyll. One group, therefore, is 

 independent in nutrition, the other dependent. 



The fungi are thought to have descended from the algse. 

 On account of the dependent habit and the loss of chlorophyll 

 there has been evolved this group of thallophytes which in 

 structure and reproduction (^Mucor, Saprolegniay often resem- 

 ble algffi. Others live as saprophytes (many bacteria, Mucor, 

 some toadstools and mushrooms, Penidllium, yeasts) ; others 

 as parasites (many bacteria, Saprolegnia, grape mildew, lilac 

 mildew, corn smut, wheat rust, tree-destroying toadstools) ; 

 others as mutualists (the lichens). The dependent habit of liv- 

 ing has often resulted in limiting parasites to a few kinds of 

 host plants without which they cannot live. It is thought that 

 members of the simplest group of thallophytes, the bacteria, 

 have descended from forms like the blue-green algse, and that as 

 they were enabled to live more and more completely as parasites 

 and saprophytes their nutritive structures became more and 

 more simple, until now they are reduced almost to the lowest 

 possible limit. It is generally supposed that different groups 

 of fungi are descended from different groups of algse. 



An important series in increasing complexity of nutritive 

 and reproductive structures is shown in the algse. In nutri- 

 tive structures we have first single-celled plants or colonies 

 (^Pleurococcus) ; then filamentous plants ( Vloihrix, CEdogonium, 

 Spirogyra); then branching filaments (FawcAen'a, Cladophora); 

 and branching filaments with leaf-like expansions (brown 

 algse). The chloroplasts, at first poorly organized, become 

 definite and often quite complex structures. The lowest algse 

 are free-floating, but a holdfast develops ( Ulothrix, (Edogonium, 

 brown and red algse) which gives relative permanence of liv- 

 ing-place. Distinct basal and apical ends and the branching 

 habit, with well-organized and well-exposed chloroplasts, pro- 

 vide a favorable organization for manufacture of foods. 



1 Only part of the types of plants that have been discussed in this text 

 are cited in this summary. 



