2] THE VARIOUS GROUPS OF PLANTS 4I 



are used commercially in the production of colloidal substances that 

 are widely employed for food and in industry. Instances are 

 ' Carrageen ' or ' Irish-moss ', used for food, and agar, which is 

 of great importance in bacteriological and allied work, though it is 

 even more extensively used in other connections. 



Myxomycetes : These are the Slime-moulds, or Mycetozoa, which, 

 as the latter name implies, exhibit animal as well as plant character- 

 istics, being indeed near the border-line of the two kingdoms, though 

 widely considered as Thallophyta. They are simple organisms 

 which in the vegetative condition (i.e., when not reproducing) 

 consist of naked, multinucleate masses of protoplasm termed 

 ' Plasmodia '. These show the animal characteristics of slowly 

 creeping movement and ingestion of food, and the plant character- 

 istics of reproduction by spores (which in some genera have cellulose 

 walls) formed in a special spore-producing organ (sporangium). 

 The vegetative plasmodium tends to shun the light and to be shape- 

 less and often several inches in diameter, growing as long as food 

 is available, though when food is scarce it may form a mere starved 

 network of living strands. Its outer layer is less liquid than the 

 inner portion and is usually devoid of nuclei ; the commonly slimy 

 appearance has led to the name of Slime-moulds. Although chloro- 

 phyll is lacking, the plasmodium may be variously and often brightly 

 coloured — most frequently yellow or brown, but sometimes orange, 

 red, black, or even greenish. 



Nutrition of Slime-moulds is primarily saprophytic, the plasmo- 

 dium living upon a variety of organic materials such as rotting wood 

 and dead leaves, apparently ingesting tiny particles of these and break- 

 ing down the complicated carbohydrates in them to simple sugars 

 which are used as food. Frequently, living bodies such as Bacteria 

 and fungal spores are ingested ; indeed. Slime-moulds can be grown 

 experimentally on an exclusive diet of appropriate Bacteria. Fruit- 

 ing bodies (sporangia) may be made when food becomes scarce ; 

 these are very various in form in different types, as indicated in 

 Fig. 10. Often they are gracefully stalked, with rounded or 

 elongated sporangia consisting of an outer membrane enclosing a 

 mass of very small uninucleate spores and, frequently, a system of 

 ramifying tubes. Sometimes almost the whole mass of protoplasm 

 becomes a single, large, spore-producing structure. The spores are 

 eventually released l3y rupture of the outer membrane and are 

 scattered by the wind. They germinate in water, each spore 



