30 PLANT LIFE. 



In young cells the protoplasm, along with the nucleus, 

 respecting which more will he said later on, almost com- 

 pletely fills the cell cavity. The differentiation usually 

 commences by the formation of an external, firmer, very 

 thin layer, which is in intimate contact with the central 

 mass ; the cell sap, that is, water containing substances in 

 solution which permeates the entire substance of the cell, 

 then collects in minute drops known as vacuoles in the 

 central mass of protoplasm. As the cell grows these vacuoles 

 increase in size, until eventually the protoplasm forms a 

 sac or layer, lining the cell-wall, and filled with watery sap. 

 The central cavity bounded by the protoplasm is called the 

 sap-cavity, which is usually traversed by strands of proto- 

 plasm in contact with the peripheral portion. These strands 

 are not permanent, but exhibit movements distinguished as 

 Circulation, and consists^ in the accumulation or diminution 

 of protoplasm forming the outer layer, or the irregular con- 

 traction and protrusion of the strands of protoplasm passing 

 from the outer layer across the cell cavity, whereby these 

 latter are constantly changing their number and arrange- 

 ment. The term streaming is applied to currents which 

 occur in the above-mentioned outer layer or strands of 

 protoplasm, and is rendered apparent by the movements 

 of enclosed granules ; such currents often move in opposite 

 directions in the same slender strand. The term Rotation 

 is applied to those cases where the entire mass of protoplasm 

 contained in a cell circulates in a rotatory manner ; this 

 movement is not always in the same direction, but rotates 

 for an irregular period of time in one direction, when the 

 action becomes reversed. Rotation can be clearly seen in 

 the stem of species of Nitella, an aquatic flowerless plant ; 

 also in the root, hairs, and other portions of some aquatic 



