24 MANUAL OF BOTANY 



for many years. In some plants, such as the DahHa (fig. 28), 

 and Orchis [fi^s. 26 and 27), the roots are the only portions 

 of the plant which are thus perennial, their stems dying down 

 to the ground yearly. In other oases the whole plant survives. 

 Perennial roots are either of woody consistence, or more or 

 less fleshy, like those of biennial plants. In the case of fleshy 

 roots such as the Dahlia and Orchis, the individual roots are 

 not in themselves perennial, but usually perish annually ; but 

 before doing so, they produce other roots from some point 

 or points of their substance ; hence, while the root system as 

 a whole is perennial, any particular portion may perish. They 

 serve a similar physiological purpose to those last named. 



Fig. 33. 



Fig. 33. Epiphytic orchidaceous plants, showing their mode of growth, 

 a, a. Aerial roots. 5, 6, Pgeudobulbs. 



Woody roots are commonly perennial in themselves, and are 

 not renewed. 



Certain peculiarities are shown by the roots of plants which 

 do not grow in the soil. Those which are to be met with in 

 water are usually long, unbranched, somewhat fleshy or succulent, 

 and devoid of hairs. They may be denominated aquatic roots. 

 Two other classes of plants also show modifications which 

 depend upon their peculiar mode of life. These are epiphytes 

 and parasites. 



Boots of Epiphytes or Air -plants. —la many of these plants 

 special aerial roots are produced (fig. 33, a, a), and as these never 

 reach the soil they cannot obtain any food from it, but must 

 draw their food entirely from substances supplied to them by 

 the air in which they are developed ; hence the name of air- 

 plants which is apphed to them. They are also called epiphytes, 



