no SHINGLE BEACHES AND THEIR FIXATION
hundreds of years, may exhibit no shoot thicker than a penholder. In this respect the plants of dormant ground, with their thick stems, stand in marked contrast (cf. p. 104).
   Before passing on to the subject of the planting of beaches, there are two matters of the greatest importance to the welfare of shingle plants that demand some notice. They are:—
   (1)	The origin and distribution of plant food or humus in
        the shingle; and
   (2)	The water problem.
Fig. 29.—Profile of Blakeney Main Beach, showing how the Sueeda Bushes cause a heaping up of the Shingle
   The Humus of Shingle.—Important to the subject of beach planting is the source from which certain indispensable elements of plant food are derived. A heap of water-worn pebbles without additions is a sterile and inhospitable substratum upon which luxuriant vegetation could not arise. This character of shingle is manifest in the case of apposition beaches—areas of shingle cut oif from the sea (cf. p. 89)—which are dependent on their own resources for the humus which accumulates in the interstices. In these beaches (which are dormant) the pioneer plants are chiefly crustaceous lichens. It is by the washing down of the products of disintegration of these that the beginnings of a vegetable mould accumulate, and the way prepared for the establishment of higher plants.
   Beaches of the spit-and-bar type, fronted by the open sea, and backed by a tidal estuary or lagoon, receive humus from two sources. From the sea comes ocean drift consisting of seaweeds, zoophytes, bits of wood, corks, and other flotsam and jetsam. These are left high on the beach, and are scattered by the winds over the crest and higher levels of the lee slope. They are met with in all stages of disintegration, and are readily assimilated by the open-textured shingle.