NATURE AND POWERS OF PROTOPLAST 



19 



Light 



in vacuoles, or deposited in the cell-walls, or thrown into inter- 

 cellular spaces; or useful substances, such as enzymes, and acids 

 for making intractable substances soluble, and nectar for alluring 

 insects, may be expelled at the surface. This isolation by the 

 protoplast of substances from the 

 sphere of its operations is called 

 excretion. 



(e) Contraction and Expan- 

 sion. — When the protoplasts are 

 not hemmed in by a cell-wallj as 

 in Euglena, the plasmodium of 

 Myxomycetes, and zoospores 

 (Fig. 10), they are found to be 

 capable of alternate contraction 

 and expansion, resulting in 

 marked change of form in 

 Euglena and plasmodia, and in 

 the thrashing back and forth of 

 the cilia of swarm spores; and 

 the circulation and rotation of 

 the cytoplasm spoken of above is 

 probably caused by contraction 

 and expansion in its different 

 parts. The power of contraction 

 and expansion is usually compre- 



FiG. 10. — I, a bit of Plasmodium of a 

 Myxomycete, arrows showing direction of 

 movement of the cytoplasm; '2, Euglena 

 viridis in different stages of contractility; 

 3, ciliated swarm-spores of Ulothrix 

 zonata; 4, diagram indicating movement 

 of swarm-spores of Ulothrix toward the 

 source of light. 



hended in the term contractility. 



(/) Perception and Response. — The protoplast is capable of per- 

 ceiving external stimuli and responding to them in a definite way. 

 Thus, when the swarm-spores of Ulothrix are mounted in a drop 

 of water for examination with the microscope they, are found to 

 swim to the side of the drop next the window, but if direct sun- 

 light is allowed to fall upon them they recede to the opposite side. 

 It is clear that they perceive the light and behave as they do on 

 account of its varying intensity; and it appears that they not only 

 perceive the light, but also the direction from which it is coming. 

 The inclining of house plants toward the window is another 



