41 8 RELATION TO ENVIRONMENT. 



contact with a body of water, or a stream. Water-roots usually 

 possess no root-hairs, or but a few, as can be seen by comparing 

 water-roots with soil-roots, or by comparing roots of plants 

 grown in water cultures. The greater body of water in contact 

 with the root and the more delicate epidermis of the root render 

 less necessary the root-hairs. The duck-meats (Lemna) are 

 good examples of plants having only water-roots. Other aquatic 

 plants like the potamogetons, etc., have true roots which grow 

 into the soil and serve to anchor the plant, but they are not devel- 

 oped as special organs of absorption, since the stem and leaves 

 largely perform this function. 



801. Holdfasts. These are organs for anchorage which are 

 not true roots. These are especially well developed in some of 

 the algae (Fucus, Laminaria, etc.). They are usually called 

 holdfasts. The holdfasts of the larger algae are mainly for 

 anchoring the plant. They do not function as absorbing organs, 

 and the structure is different from that of true roots. 



802. Haustoria or suckers is a name applied to another kind 

 of holdfast employed by parasitic plants. In the dodder the 

 haustorium penetrates the tissue of the host (the plant on which 

 the parasite grows), and besides furnishing a means of attach- 

 ment, it serves as an absorbing organ by means of which the 

 parasite absorbs food from its host. The parasitic fungi like 

 the powdery mildews which grow on the surface of their hosts 

 have simple haustoria which serve both as organs of attachment 

 and absorption, while in the rusts which grow in the interior of 

 their hosts the haustoria are merely absorbing organs. 



803. Rootlets, or rhizoids. Many of the algas, liverworts and 

 mosses have slender, hair-like organs of attachment and absorp- 

 tion. These plants do not have true roots. Because of the 

 slender form and small size of these organs, they are called 

 rhizoids, or rootlets. In form many of them resemble the root- 

 hairs of higher plants. 



