Biology of Higher Plants — Anatomy and Physiology 241 



very little, if any, movement. The plant hormones may be transported 

 in the following ways : ( 1 ) by diffusion, ( 2 ) by protoplasmic streaming, 

 (3) by the transportation or circulatory system of the plant, if such is 

 present, (4) by an electrical phenomenon in which they are moved 

 toward a positively charged pole because of changes in electrical poten- 

 tial within the plant. A similar phenomenon in animals has been sug- 

 gested by recent experimental evidence. In spite of the fact that plant 

 hormones can be extracted from plants, there is no chemical test which 

 provides a simple and efficient means of qualitative and quantitative 

 detection of the minute amounts of them in living plants. However, 

 certain physiologic methods are now being perfected by means of which 

 the hormone concentration can be determined. 



XI. GROWTH OF PLANTS, POLARITY, MORPHOGENESIS 



Plants increase in size by mitosis (cell division) or by an increase of 

 the size of the cells without increasing the number. In many instances 

 growth is probably the result of both these phenomena occurring at the 

 same time. There is a limit to the size to which a cell can grow and 

 normally carry on its metabolic activities. After a certain size is 

 reached, mitosis must occur and the two resulting cells must increase in 

 size by assimilating foods brought to them. It is not known precisely 

 how the living protoplasm in these plant cells assimilates this food. 

 Undoubtedly, the various food elements are built up and held together 

 by energy supplied to the plant, principally through oxidation of food 

 materials. The actual rate of growth of a particular plant, or any of 

 its parts, is influenced by such factors as (1) the specific inheritance of 

 those cells, (2) the quantity and quality of available foods, (3) the age 

 of the plant, (4) the amount of available oxygen, and (5) the presence 

 of specific plant hormones. 



Generally speaking, plant growth hormones bring about growth if 

 such conditions as water supply and foods are satisfactory. It is be- 

 lieved that no plant growths can take place without the presence of the 

 specific plant hormones previously described. The hormone auxin in 

 minute quantities promotes the elongation of cells (stretching) in the 

 direction of the long axis of an organ, such as a stem or branch. In 

 this case auxin is said to promote "polarized growth"; that is, growth 

 in length rather than in another direction. This is particularly true in 

 younger tissues. After tissues have reached a certain age, growth occurs 

 in such a manner that the tissue increases in diameter. In all instances 

 growth is dependent on plant hormones. 



