INFLUENCE OF ENVIRONMENT 275 



gradual. As in Eucalyptus it is not uncommon to find 

 a reversion to the original leaf-form on young shoots of 

 the older trees. 



Similar in their behavior to desert plants are the 

 " Halophytes, " those growing along the seashore or in 

 salt marshes. Thus the sea-rocket (Cakile), samphire 

 (Salicornia), ice-plant (Mesembryanthemum), and other 

 maritime plants show these peculiarities. These plants 

 have fleshy stems and leaves and can live with very 

 little water. The explanation of this peculiarity in 

 plants growing where there seems to be an abun- 

 dance of water, has been thought to be the fact that 

 the separation of the water from the salt solution is 

 difficult, and, moreover, the accumulation of salt within 

 the tissues of the plants, if free transpiration of water 

 from the surface took place, would be injurious to the 

 plant. 



EPIPHYTES 



Under the name Epiphytes are included those plants 

 which grow attached to others, but are not parasites. 

 While these epiphytes usually grow upon trees or other 

 plants, not infrequently they may attach themselves to 

 rocks or other inanimate objects. Epiphytic plants are 

 most abundant in the moist, hot regions of the tropics, 

 but are by no means confined to these, since many 

 mosses, lichens, and liverworts which occur plentifully 

 in the temperate or even arctic regions, may be prop- 

 erly classed as epiphytes. Of the ferns and flower- 

 ing plants, however, very few epiphytic types occur 

 outside the tropics, though there they form a most 

 characteristic feature of the vegetation. 



