281 



ever, is not dry throughout. The sand to within a few centimeters 

 of the surface is moist. The layer of dry sand which acts as a very 

 good non-conductor of heat prevents the entire desiccation of a dune. 

 Because of this, vegetation there is possible. 



Adaptations of the Vegetation. — The characteristic adaptation of 

 sand-dune plants is found in the extreme development of the root 

 system in comparison with the aerial parts. To meet the constant 

 shifting of the sand, which may uncover the roots, they are capable 

 of producing adventitious shoots. Because of this, the plant can 

 sometimes move a considerable distance in keeping pace with the 

 sand. Sand-dune plants usually cover quite a little ground, and thus 

 protect themselves from exposure of their roots because of the blow- 

 ing sand. The grasses that inhabit the dunes are perennials, and they 

 are frecjuently tufted. The mere presence of some of these grasses 

 on the upper beach may often be the starting-point of a dune. 



The aerial parts are clearly developed in response to the extremely 

 xerophytic habitat. The leaves are firm in texture, with stomata 

 w^ell protected by the position of the leaves or by a protecting cover- 

 ing of hairs. Often the leaves are long and narrow and curled or 

 folded to reduce transpiration. The inflorescence is frequently pro- 

 tected in the upper sheaths until it is virtually fully ready for polli- 

 nation. 



Plants as_ Dune Builders. — (Cf. Cowles, 1899:175 et seq.) 

 Plants may live on a dune and yet add nothing to the life of a dune. 

 They will accumulate sand during a season and form miniature or 

 embryonic dunes, but as soon as the plants die down in autumn the 

 sand is again mobile. Such dunes very seldom last during the winter, 

 although many of them are formed during the growing season. They 

 are the "annual dunes" of Cowles (1899:177). To endure from 

 season to season a dune must be fixed by perennials, particularly of 

 the group known as sand-binders. It is well known that owing to 

 the persistence of the vegetative parts in winter such plants have con- 

 siderable ability to prevent sand from shifting. For a dune to grow 

 larger the sand-binder must be able to respond easily to changing 

 conditions ; and it must not be killed by exposure of its root system 

 nor by the burial of its stem. To make the dune more extensive it 

 must be able to spread radially by rhizome development, thus devel- 

 oping the dune in expanse at the same time that the upward growth 

 of the stems is developing it in altitude. 



Location in the Beach Area. — The sand-dunes occur a little be- 

 yond the limit of winter wave-action. They are more general in oc- 

 currence and better developed in constructive beaches. Nowhere in 



