168 



PLANT PHYSIOLOGY AND ECOLOGY 



Experiment 53. Structure of floating plants. Make an outline drawing 

 in which Nymphcm and Lemna are contrasted. Draw a cross-section 

 of the floating leaf and compare it with a similar section of a Lemna 

 thallus. 



187. Submerged plants 



duced in submerged forms. 



Stem and root are both greatly re- 

 This is due to the fact that absorption 

 is no longer carried on solely by 

 the root, but by the whole stem 

 and leaf surface as well. The 

 greater density of water as com- 

 pared with air also renders sup- 

 port less necessary, and the stems 

 are unusually long and slender, 

 with poorly developed bundles. 

 The leaves are greatly reduced 



Fig. 56. Cross-section of a sub- in size and thickness, and in form 

 merged (3a) and an aerial (36) ,1 ., , n r i- 



leaf of Callitrwhe bifida. they are ribbon-hke, Imear, cylm- 



drical, or finely dissected. These 



changes seem chiefly to serve the purpose of increasing the surface 



for absorption, especially of gases, and for receiving the diffuse 



light rays. Stomata are sometimes present, but they are always 



functionless. The chlorenchym is essentially that of a shade leaf. 



In the few cases where palisade and sponge tissues are present, 



they are doubtless relics of a former structure. The air chambers 



are either much reduced or entirely lacking. \hen present, they 



probably serve as reservoirs for air obtained from the water. 



Experiment 54. Structure of submerged plants. Make an outline 

 drawing of the shoot and root system of Ceratophyllum, Myriophyllum, 

 or Philotria. Draw the entire leaf in cross-section. 



Make a detailed comiDarison by means of a table of the form and 

 leaf structure of the various types of hydrophytes. 



188. Bog plants. Many plants of bogs, ponds, banks of streams, 

 etc., have the appearance of xerophytes, in spite of the fact that 

 they grow in water. Their leaves are more or less reduced, and 

 are sometimes lacking. The cuticle is thickened and the palisade 

 tissue well developed. The usual explanation of " bog xero- 

 phytes " is that they are caused by humic acids in the water, 

 which hinder the absorption and aeration of the roots. In other 

 words, the available water is thought to be small, though the 



