VITAL PROPERTIES OF CELLS 15 



fundamentally a metabolic process, and in essence consists of 'a progres- 

 sive differentiation of complex and specialized structures and functions 

 from relatively simple and generalized beginnings' (Conklin). 



(2) Irritability. Irritability, or sensitivity, is a fundamental or 

 general property of protoplasm. It is characterized by a capacity to 

 receive and make response to stimuli, by changes of vital processe&r Its 

 prerequisite is the protoplasmic property of conductivity, and its expres- 

 sion in many instances depends upon the property of contractility. In 

 a comprehensive sense, stimulus is every alteration in the external vital 

 condition (Verworn). The reaction to stimuli may exhibit itself in 

 one of three modes: functional, nutritive, and formative (Verworn). 



FIG. 20. SUCCESSIVE STAGES IN THE MOVEMENT OF AN AMEBA. 

 The cells contain a nucleus, a contractile vacuole and protoplasmic granules. 

 (After Verworn.) 



Stimuli are of various sorts, e.g., thermal, mechanical, chemical, photic, 

 solar, galvanic (or electric), fluid, current, gravity. The simplest re- 

 sulting reactions, expressed in unicellular forms in terms of orientation 

 (tropism) or contact (taxis) represented by automatic responses or 

 reflexes in higher forms are respectively thermotropism, stereotropism 

 (barotaxis; thigmotaxis), chemotropism, phototropism, heliotropism, 

 galvanotropism, hydrotropism, rheotropism, and geotropism. Response 

 may be either toward or away from the source of stimulus. In the case 

 of the electric current or water currents, for example, simple organisms 

 may orient themselves, or protoplasm may move, in line with or opposite 

 to the current; these opposite reactions are called positive and negative 

 tropisms respectively. Responses involve fundamentally metabolic 

 changes. 



(3) Contractility. Motion results from response to certain stim- 

 uli, that is, by reason of irritability; and it is dependent upon the vital 

 phenomenon of contractility. Motion is of various types, predominant 

 among which are (a) ameboid, (b) ciliary, (c) molecular, (d) circula- 

 tory (streaming; protoplasmic), and (e) muscular. 



