72 PLANT LIFE. 



spontaneous movement, due to the presence of cilia like the 

 typical Protozoa, but differing from the latter in being able 

 to add to its bulk, and derive its energy directly from in- 

 organic matter. Along with this change is the development 

 of cellulose used for protective purposes in the form of 

 cell- walls. As already said, locomotion is still possessed by 

 some of the simpler forms of plant life, but owing to the 

 fact that the inorganic food required by the members of this 

 new line of departure exists everywhere, the power of loco- 

 motion became lost, and we find the great majority of 

 plants permanently fixed, and it is only when the group has 

 become broken up into highly specialized branches that we 

 meet with plants possessed of migratory habits, necessitated 

 by the requirement of special kinds of food present in only 

 small quantities at any one spot. The mode of migration is 

 not however effected by the animal method, but by the 

 constant extension of those portions of the plant that are 

 destined to develop into the vegetative parts during succes- 

 sive seasons, away from the locality previously occupied. 

 The common bracken illustrates this method of migration, 

 the growing-point of the underground stem extending hori- 

 zontally from year to year, and producing new roots and 

 a frond in a new locality. Our common orchids and many 

 other plants move from place to place in a similar manner, 

 and even forest trees that appear to be permanently fixed in 

 one place, follow the same idea by constantly extending 

 their roots into new soil. As already pointed out, the 

 inorganic food required by plants, on account of its general 

 distribution allows of a wide range, limited only by the 

 presence of water, which is necessary for the solution of the 

 various salts derived from the soil or rock on which the 

 plant is growing. Nevertheless this feature more than any 



