84 PLANT-LIFE ON LAND [CH. vi 



in the plant the complete enclosure or encystment of 

 the primitively naked protoplast by a cell-wall pre- 

 sents no serious obstacle, provided that the cell- wall 

 be permeable to the molecules of the food to be taken 

 in, these being in the condition of solution in water. 

 But encystment brings great mechanical advantages, 

 and is indeed the chief means by which the construc- 

 tion of the large tissue-masses of the plant becomes 

 mechanically possible. So long as the wall is 

 sufficiently permeable, there is no need for any part 

 of the protoplast to be freely exposed, and as a 

 matter of fact, except among the simplest organisms, 

 or the reproductive cells of those which . are more 

 complex, the cells of the plant-body are habitually 

 encysted (Fig. 18). But the cell- wall which is thus 

 a general feature in all but the simplest plants 

 presents at once an obstacle to free movement. The 

 protoplast is fettered by its armour more seriously 

 than any mediaeval knight. It has sold its birth- 

 right of free external movement for the pottage 

 of mechanical protection and structural stability. 

 Among the simpler aquatic organisms many naked 

 plant-forms are found, which like the simpler animals 

 are capable of active movement. But there are 

 others in which the movement is only an occasional 

 incident connected with propagation, others again in 

 which the encysted state is permanent. These may 

 be held to illustrate the sort of steps which have led 



