PLAXT PHYSIOLOGY 303 



it ceases death ensues sooner or later. By this process 

 energy, stored up by the processes of nutrition, is liberated, 

 and with this liberated energy the plant works. It may be 

 said that oxygen seems to have the power of arousing pro- 

 toplasm to activity. 



It is not sufficient for the air containing oxygen to come 

 in contact merely with the outer surface of a complex plant, 

 as its absorption and transfer would be too slow. There 

 must be an " internal atmosphere " in contact with the 

 living cells. This is provided for by the intercellular 

 spaces, which form a labyrinthine system of passageways, 

 opening at the surface through stomata and lenticels (pores 

 through bark). In this internal atmosphere the exchange 

 of oxygen and carbon dioxide is effected, the oxygen being 

 renewed by diffusion from the outside, and the carbon 

 dioxide finally escaping by diffusion to the outside. 



ilOTEMENT 



168. Introductory. — In addition to movements of mate- 

 rial, as described above, plants execute movements depend- 

 ent upon the activity of protoplasm, which result in change 

 of position. Xaked masses of protoplasm, as the Plas- 

 modium of slime-moulds (see § 51), advance with a sliding, 

 snail-like movement upon surfaces ; zoospores and ciliated 

 sperms swim freely about by means of motile cilia ; while 

 many low plants, as Bacteria (§ 52), Diatoms (S 31), OhcU- 

 laria (§ 20), etc., have the power of locomotion. 



When the protoplasm is confined within rigid walls and 

 tissues, as in most plants, the power of locomotion usually 

 disappears, and the plants are fixed ; but within active cells 

 the protoplasm continues to move, streaming back and 

 forth and about within the confines of the cell. 



In the case of complex plants, however, another kind 

 of movement is apparent, by which parts are moved and 

 variously directed, sometimes slowly, sometimes with great 



