86 PLANT-LIFE ON LAND [CH. 



from the primitive freedom of the protoplast to the 

 bondage of encystment. 



The protoplast thus encysted remains isolated 

 only in the simplest forms. In all the more complex 

 the plant-body is made up of many, it may be of 

 millions of individual cells, derived ultimately by 

 division from a common source. Each is united with 

 its neighbours so as to constitute a tissue, or tissue- 

 system. The bondage of each encysted cell applies 

 collectively to the whole mass of them, and a large, 

 relatively immobile plant-body is the result. It is 

 no wonder that with tissues thus constituted the 

 movements of plants even in the young state are slow, 

 and act within narrow limits : and that as the tissues 

 become mature and their cell-walls thicken, the parts 

 composed of them become rigid, and lose the power 

 of movement altogether. 



The nutrition of such encysted cells or cell-masses, 

 in the case of the Algae which inhabit fresh or salt 

 water, may take place at any point of their surface 

 which is exposed to the surrounding medium. Such 

 Algae are in simple cases suspended without attach- 

 ment, and constitute the large proportion of the so- 

 called "plankton," or floating life. They may be 

 collected in quantity from the surface of salt or fresh 

 water, in tow-nets of muslin drawn in the wake of a 

 boat. The collection will be found to consist for the 

 most part of relatively simple organisms of small size, 



