Environment and Adaptation 197 



The hydrophytes are sometimes characterized by 

 the poor development of roots, which may actually 

 be absent ia some of them. Most of the hydro- 

 phytes are herbaceous plants, and the stems and leaf 

 stalks are provided with very large air spaces. 



MESOPHYTES 



Where plants are provided with an adequate but 

 not excessive amount of water, they develop a 

 perfect root-system, and an ample expanse of green 

 tissue, either in the form of a flat thallus, or, in the 

 higher plants, of leaves of various kinds. The size 

 of the leaf is to a certain extent dependent upon the 

 intensity of light and upon the amount of moisture, 

 the two often being in inverse ratio. Other things 

 being equal, transpiration is less active in the shade 

 than in the full light, and shade plants normally 

 exhibit a much larger leaf surface than those ex- 

 posed to full sunlight. The difference is very evi- 

 dent in plants of the same species, or even in the 

 same individual, and can be readily enough demon- 

 strated. If the light is completely excluded, or is 

 too weak for photosynthesis, there usually is a de- 

 generation of the leaf lamina, which may be almost 

 completely suppressed, a fact familiarly demon- 

 strated by the blanched and shrunken leaves of a 

 plant sprouted in the dark. 



Types of Leaves in Mesophytes. The increase 

 in the extent of the leaf surface may be effected in 



