GROWTH AND MOVEMENT 



445 



are often included in the animal kingdom (see p. i). The plasmodium 

 is a naked mass of protoplasm (sometimes like a thin cake, often a richly 

 anastomosed network), which during its vegetative period lives in wet 

 places among decaying wood, leaves, etc. The creeping is accomplished 

 by the protrusion of marginal lobes of the protoplast along one side, and 

 toward these the rest slowly flows^ In this way the whole mass advances 

 in a definite direction, which is frequently changed and is subject to 

 control by external agents. Thus, by varying the temperature, the mois- 

 ture, or the illumination, the plasmodium may be made to creep in one 

 direction or another. Its response to these stimuli, however, differs 

 with its own stage of development. Whereas during a considerable 

 vegetative period it avoids light and drier places, later it creeps out from 

 the substratum and ascends to drier and exposed situations, where it 

 produces sporangia with a casing and framework of cellulose and a 

 multitude of spores. 



Excretory movements. — Excretory movements are executed by some 

 diatoms and desmids, and those of Oscillatoria and Spirogyra are 

 probably of this sort. The diatoms and desmids forcibly excrete muci- 

 lage through slits or pores in the wall against the substratum (a glass slide, 

 the wall of an aquarium, the bottom of a pool, or the surface of a water 

 plant) over which they creep slowly with a majestic 

 and mysterious motion, which is not yet fully under- 

 stood (see also p. 451)- 



Ciliary movement. — The more rapid movements 

 are called ciliary, because executed by the lashing of 

 slender threads of protoplasm through the water, in 

 which alone such organisms can move. The motile 

 threads are known as cilia or fiagella} They arise 

 from different places on the protoplast, often at the 

 pointed apex or along a band, where the special 

 organ which produces them, the blepharoplast, is Fig. 678. — Swarm 

 located (fig. 678). The flagellates (unicellular organ- ^trtwo^^cmatrk™g 

 isms of uncertain relationship, p. 20), bacteria, the from a blepharoplast 

 zoospores and gametes of certain algae and fungi, ^oit.- After tTmber- 

 and the sperms of bryophytes, pteridophytes, and lake. 



' No constant distinction can be made between cilia, which are typically short, hair- 

 like, and numerous, and fla'^ella, which are long, whiplike, and few (1-4) for each cell. 

 Yet a cell sometimes has a single cilium, or two, and flagella are numerous on the sperms 

 of ferns. 



