The Scottish Naturalist. 



291 



vegetation becomes more luxuriant, till, finally, it is submerged by 

 great floods only. These bring to it vegetable humus, till at length 

 the shingle proper forms only the foundation of a rich haugh or 

 island, covered with a luxuriant undergrowth of herbaceous plants, 

 and not unfrequently adorned with shrubs and trees. But unless 

 man, by building jetties, protecting the banks, or forming em- 

 bankments, interrupts the natural course of events, sooner or 

 later the river will destroy all that it has created, only, however, to 

 repeat the construction in some other pare of its course. 



In the following remarks on the flora of the shingles and stanirs, 

 I purpose confining myself chiefly to those of the middle part of 

 the course of the river Tay; but what applies to it will also apply 

 in great measure to all rapid Scottish rivers whose sources are in 

 the loftier mountains. 



Taking in the first place a shingle or stanir in course of forma- 

 tion — namely, when it is dry during a comparatively short period 

 of the year only. From such shingles we can learn two pieces 

 of information, first, as to the plants whose seeds are conveyed 

 entirely or to a very great extent by water, and second, as to the 

 species which can grow and flourish in a soil which, even in the 

 most favourable circumstances, consists only of coarse gravel, and 

 more often for the chief part of stones, 2 or 3 inches or more in 

 diameter. That water is the only agent which brings the seeds 

 (or, very rarely, rooted plants) to such spots is evident, because 

 at the time the species in question were mostly in seed, the shingle 

 itself was submerged ; or, if it was not, seeds brought by the wind 

 would be so superficially deposited that the first flood would 

 remove them. I am, of course, speaking of a shingle which has 

 just arrived at a condition to bear its first crop. After it has 

 carried one the seeds thereof may more readily find a permanent 

 resting-place, though, as all the upper layers of stones are in motion 

 during every flood, it is probable that in the earlier years of the 

 life of a shingle none of the seeds produced on it remain there. 



I have thought that much instruction might be derivedif a definite 

 space, — say fifty yards square, — of a young shingle were mapped 

 out, and a careful census taken of all the plants which spring up 

 in it in course of one season ; but I have not been able to put 

 this idea into execution. On such a shingle the first plants that 

 are at all conspicuous are some of the weeds of cultivation, e.g. 

 Brassica SinaJ>is } Chrysanthemum segetum i Matricaria modora, 



