72 RELATION OF STRUCTURE AND FUNCTION 



endogenous origin contrasts with tire superficial origin of the 

 branches of the stem, and can be related to a need for protection, 

 until the young root has developed sufficiently to withstand the 

 resistance of the soil. 



It is now possible to inquire how the structure of the root 

 is related to its two principal functions, viz. absorption of water 

 from the soil and the anchorage of the plant. Absorption of 

 water takes place through those regions in which the piliferous 

 layer is still intact and living. The outgrowth of its cells into 

 root-hairs, which are the chief organs of absorption, affords an 

 enormously increased surface over which the latter can take 

 place. The size and number of the root-hairs tend to decrease 

 with increasing wetness of the soil, and in some marsh-plants 

 [e.g. Marsh Marigold) and a considerable number of aquatics 

 they may be altogether absent. Here, owing to the high water- 

 content of the soil, an enlargement of the absorbing surface is 

 unnecessary. The root-hairs not only function in absorbing 

 the water with its dissolved mineral salts, but themselves play 

 a part in rendering substances in the soil available. The older 

 part of the root which does not absorb is amply protected by 

 the exodermis. 



Many plants characteristic of soils rich in humus exhibit an 

 intimate relation of fungal threads with their roots or other 

 underground organs. In some cases these threads form a dense 

 weft over the whole surface {ectotrophic mycorrhiza , e.g. Beech, 

 Birch, etc.), and appear to replace the absent root-hairs. In 

 others the Fungus can be seen occupying a definite zone within 

 the cortex [endotrophic mycorrhiza, e.g. Bird's Nest Orchid, 

 (Fig. 31, i1/. ), Heather, etc.]. The advantage of association with 

 the Fungus would appear to depend mainh' on the po\ver of the 

 latter to break down and absorb the organic material which is 

 then in part utilised by the Flowering Plant. 



The root is suited to its anchoring function bv its more or 

 less extensive branching, and the central location of the me- 

 chanical elements which enable it to withstand the pulling strain 

 to which it is subjected when the shoot sways to and fro in the 

 wind. In most roots the mechanical tissue is constituted merelv 

 by the xylem, as well as by the pith ^^■hen the latter is thick- 

 wahed, but in the Pea, Bean, and other Lcguminos;e groups of 



