8o8 



ECOLOGY 



1118 1119 

 FIGS. 1118, 1119. Liver- 

 wort gemmae: 1118, a thallus 

 lobe of Lunnlaria vulgaris, 

 bearing a crescentic cupule in 

 which are numerous gemmae; 

 1119, a single gemma from a 

 cupule of Marchantia polymor- 

 pha; considerably magnified. 



by asexual spores. Gemmation may be defined as the organization of 

 vegetative buds that readily become detached from the parent plant; 



such disseminules are called gemmae. The 

 simplest gemmae are unicellular, and they 

 differ from asexual spores chiefly in the 

 absence of a protective wall and of the 

 resting period usually associated therewith, 

 although there exist all intergradations be- 

 tween the two. Simple gemmae of this char- 

 acter are found in yeast (figs. 168-173), and 

 also in Mucor and in Vaucheria geminata. 

 Some species of liverworts also have uni- 

 cellular gemmae on the leaf margins; in 

 Aneura the gemmae are two-celled, and in 

 Marchantia and Lunularia they are multicellular and are borne in clus- 

 ters in special cupules (figs. 1118, 1119). 



Multicellular gemmae occur also on various mosses (as Georgia pellucida), on 

 fern prothallia, and on some algae (as Sphacelaria and Char a). The soredia of 

 lichens (figs. 1114-1116) also may be classed with gemmae. Certain structures in 

 the vascular plants, such as the gemmae of Lycopodium, the leaf bulbils of ferns, 

 the stem bulbils of lilies, and the inflorescence bulbils of the onion (p. 902) are com- 

 parable to the gemmae of the lower plants. 



Sclerotia. In autumn the mycelium of the ergot fungus (Claviceps) becomes 

 enveloped in a dense and relatively impermeable protective layer of dark, thick- 

 walled cells, within which the vegetative hyphae remain 

 dormant over winter ; the entire structure, which is richly 

 packed with food, is called a sclerotium (5g. 1120). In 

 spring the sclerotium germinates, and ordinary vegetative 

 activity is resumed. Somewhat similar to the sclerotia of 

 ergot are those of Peziza sclerotiorum and of various other 

 fungi. Many fungi (e.g. Sclerotinia) have subterranean 

 tuber-like sclerotia richly packed with food, which endure 

 through unfavorable periods, and other forms have tough 

 sclerotial strands resembling shoestrings. In the myxomy- 

 cetes the plasmodium or a part of it may become encysted 

 into a sclerotial mass and remain dormant even for years. 

 While strictly vegetative tissue is involved in the formation 

 of sclerotia, they agree with spores and seeds in being formed 

 at the close of vegetative periods and in being fitted for 

 existence in a dormant state during severe periods. Com- 

 parable to sclerotia are the resting cells of bacteria, the thick- 

 walled resting cells of Nostoc which are closely packed with food, the starchy tubers 

 of Chara, and the subterranean resting buds of liverworts and mosses (fig. 251). 



FIG. 1 1 20. A 

 sclerotium (5) of the 

 ergot fungus (Clavi- 

 ceps purpurea), grow- 

 ing from a spikelet of 

 the sand-reed (Am- 

 mophila arenaria). 



