38 



PHYSIOLOGY OF MUSCLE AND NERVE 



fact that the tension or turgor existing in the sap-sac is brought to bear upon the ' 

 elastic wall of the cell with the result that the size of the latter is either increased 

 or diminished. In many plants these changes in the turgescence occur very sud- 

 denly and either spontaneously or in consequence of a stimulus of some kind. As 

 an example of this type of motion might be mentioned the folding up and drooping 

 of the leaflets of the sensitive plants (mimosa pudica), when touched or when ex- 

 posed to low intensities of light. Sunlight, on the other hand, causes them to 

 unfold and to erect their stems and leaflets. A similar phenomenon is exhibited 

 by the insect-catching flowers of the carnivorous plants. 



Movements by changes in the specific gravity may be observed in certain 

 radiolaria. Ordinarily these organisms are heavier than water and creep along 

 the bottom of stagnant pools. They are capable, however, of rising to the surface 

 by generating small bubbles of carbon dioxid which are deposited among their 



protoplasmic streamers. At the surface this gas 

 is quickly discharged. In consequence of the in- 

 crease in their specific gravity then resulting they , 

 again sink to the bottom. 



Movements by secretion result in algae and 

 oscillarise and are produced by the projection from 

 their bodies of a sticky liquid which adheres to 

 the surface of the receptacle. As a result of this 

 secretion the body of the organism is slowly forced 

 forward in a definite direction. 



Movements by growth are very general and 

 occur whenever a cell increases its mass. But as 

 the ordinary processes of assimilation are slow, 

 the detection of this movement often necessitates 

 a comparison of conditions at different periods of 

 the life of the organism. Many seedlings display 

 a more perceptible growth. Moreover, many 

 seeds and fruits require only the slightest touch 

 to make them burst and to discharge their con- 

 tents. In these cases the mechanical energy de- 

 veloped by growth has been stored, and has placed 

 the capsular investment under a high degree of 

 tension. 



The alternate contraction and expansion of a 

 mass of protoplasm means that it assumes a 

 rounded shape during the former phase and a 

 flat outline during the latter. Its surface, there- 

 fore, undergoes constant changes, but naturally, 

 only those organisms can display this phenomenon 

 in a plastic manner which possess a liquid consistency. We have previously seen 

 that this characteristic is universal among living substance, but whether an organ- 

 ism as a whole is motile, depends, of course, upon the character of its framework 

 which may or may not be sufficiently yielding to permit the contraction of its 

 protoplasm. Three types of structures are evidently well adapted for this pur- 

 pose, namely, (a) small masses of living substance which are not surrounded by a 

 distinct cell wall, (6) hair-like protoplasmic processes with which many cells are 

 beset, and (c) the muscle cell as it appears in striated, non-striated and cardiac 

 tissue. 



Ameboid Movement. When placed upon a slide under the micro- 

 scope, an ameba-cell appears as a gray droplet embracing a nucleus 

 and contracting vacuole. Its central portion, consisting of endoplasm, 

 contains as a rule a number of granules, while its peripheral zone, or 

 exoplasm, is more or less hyaline. When kept under optimum condi- 



FlG. 8. DlATOMAE, 



SHOWING PROTRUSION OF Mu- 

 cous MATERIAL. (Verworn.) 



