CHAPTEE V. 

 PLANT PHYSIOLOGY. 



126. Definition. — Plants not only have members and 

 organs, which are composed of cells, tissues and tissue- 

 systems, but in addition, they have activities, sometimes 

 pertaining to the whole plant, sometimes to the members, 

 the tissues, or the cells. A study of these activities is 

 Physiology. 



127. Divisions of Physiology. — The activities of plants 

 may be considered under five heads, viz. ; Nutrition, 

 Growth, The Physics of Vegetation, Plant Movements, and 

 Eeproduction. 



NUTRITIOK. 



128. Absorption. — Nutrition includes all those activities 

 which have to do with the supply of matter to meet the 

 wants of living cells. The life of a cell involves the use 

 of matter, and as long as a cell is living it must have a 

 continual supply of certain substances. Accordingly we 

 find that every mass of living protoplasm under favorable 

 conditions is continually absorbing watery solutions. Im- 

 bibition is one of the most pronounced of the properties of 

 living protoplasm, and its absence is one of the marked 

 distinctions between living and dead cells. Along with 

 the water thus absorbed, are taken in the various sub- 

 stances dissolved in it; these may have been solids dis- 

 solved in the water, or liquids, or even gases. It appears, 



74 



