56 BOTANY part i 



vesicle is a contractile vacuole. The presence of such a contractile 

 vacuole in an organism was formerly considered a certain indication 

 of its animal nature. Now, however, contractile vacuoles have been 

 observed in the swarm-spores of many green Algae, of whose vegetable 

 nature there can be no doubt. 



The swarm-spores of the Myxomycetes soon lose this characteristic 

 swarm-movement, draw in their flagella, and pass into the amoeba 

 stage of their development, in which, like animal amoebae, they 

 assume irregular, constantly changing shapes, and are capable of 

 performing only amoeboid creeping movements. In- the case of 

 Chondrioderma difforme, a Myxomycete of frequent occurrence in 

 rotting parts of plants (Fig. 58), a number of the amoebae eventually 

 collect together (/) and coalesce. In this way, as is also the case 

 with most other Myxomycetes, the amoebae ultimately give rise to 

 a Plasmodium («) (^^). 



Although each one of the amoebae is so small that it can only be 

 seen with the aid of a microscope, the plasmodium into which they 

 become united may attain a considerable size. 



The cytoplasm, both of the single amoeba and of the plasmodium, 

 consists of a clear ground substance, through which granules are dis- 

 tributed. This substance is of the consistence of a tenacious fluid ; 

 its superficial region is denser and free from granules, while these are 

 numerous in the less dense central portion. The granules enable the 

 internal streaming movements of the cytoplasm to be recognised. 

 The currents are constantly changing their direction, moving either 

 towards or away from the margin. The formation and withdrawal 

 of processes of the margin stand in relation to the direction of the 

 currents. AVhen naked masses of protoplasm such as these plasmodia 

 encounter foreign bodies, they can enclose them in vacuoles, and, 

 when of use as food, digest them. 



Deprived of its component water the protoplasm becomes hard and 

 tenacious, and, Avithout losing its vitality, ceases to perform any of 

 its vital functions until again awakened into activity by a fresh supply 

 of water. In case of a scarcity of water the plasmodia of the INIyxo- 

 mycetes may form sclerotia, that is, masses of resting protoplasm of 

 an almost wax-like consistency. Months and indeed sometimes years 

 afterwards, it is possible for such sclerotia, if M'ater be supplied, to 

 again produce motile plasmodia. Similarly, in seeds kept for a long 

 time the protoplasm consolidates into a hard mass, which may be 

 easily cut Avith a knife, while the nuclei will be found to have shruidc 

 and lost their original shape. Nevertheless the protoplasts, after 

 absorbing water, ma}?^ return again to a condition of activity. 



Protoplasm is not a simple substance chemically ; it consists rather 

 of numerous diflierent components which are subject to continual change. 

 Since all)uminous substances are always present, protoi)lasm always gives 

 a proteid reaction ; when incinerated, fumes of ammonia are given off. 



