4i6 



NA TURE 



[June 2, 1910 



whether peaty or non-peaty, rich or poor in plant food- 

 salts, &c. ; and these characters very largely depend upon 

 the physiographical features of the surrounding country. 

 When the combination of such factors respecting any loch 

 is known, the plants likely to be found there may be 



flora, but between the flora and fauna as well ; and it is 

 at present impossible to sefc down a complete satisfactory 

 statement of the ecology of any single group of aquatic 

 plants. We must, therefore, for the moment leave the 

 final generalisation of the causes which govern the dis- 

 tribution of plants, and content our- 

 selves with the routine work 6\ taking 

 evidence of that which occurs, not being 

 too eager to surround ourselves with 

 metaphysical hypotheses which seek to 

 explain the observed phenomenon by a 

 noumenon, nor to cloak our ignorance 

 and delude our senses with vague con- 

 cepts that transcend actual demonstra- 

 tion, and, when analysed, explain to our 

 intelligence nothing whatever. 



Again, the restriction of certain plants 

 to particular localities may be accounted 

 for by observing that they are ill 

 adapted for any mode of dispersal to 

 which they are likely to be subjected ; 

 it is then difficult to understand their 

 introduction to their present situation. 

 It is not easily explained why Equisetunt 

 limosu7n, Carex rostrata, Phragmites 

 communis, and others, should be so 

 widely distributed about the margins of 

 all kinds of lochs, whereas Cladiuni 

 Mariscus, an equally dominant species, 

 should be restricted, in the areas under 

 discussion, to a few places in Wigtovi'n- 

 shire. When the subscience of plant- 

 ecology has taught us the full facts re- 

 garding the relationship existing between 

 organism and environment, then shall we 

 be able to generalise sets of phenomena 

 regarding the geographical distribution 

 of water plants to some useful purpose. 

 The study of the plants of the lochs suggests that 

 aquatic plants have not always had their origin frcr 

 terrestrial forms that had been forced into the water b 

 more robust competitors on the land, as is sometimes 

 stated, but, more probably, because certain mutable forms 



Fic. 3. — An example of a small, somewhat wind-sheltered semi-highland loch the western margin of 

 which has a zone of marsh vegetation— Loch Dow, Cleish Hills. View of ihe west side from the 

 south end, looking north-west, showing Carex rostrata bordering the west side of the loch. The 

 darker patch standing out of the water on the right is an association oi Equisetunt liinosum. 



roughly indicated, but this apparent simplicity is frequently 

 modified by other agencies. 



The laws which govern the geographical distribution of 

 aquatic plants cannot be fully understood until science has 

 revealed more facts regarding the ecology of the plants 

 than it at present possesses ; it is there- 

 fore futile to attempt the deduction of 

 general laws, with only an inadequate 

 knowledge of the phenomena to be 

 generalised. During the last great 

 Glacial epoch it is certain that all forms 

 of the higher plants were banished from 

 the greater portion of Scotland. To- 

 wards the end of that era, as the mantle 

 of ice and snow began to retreat, so 

 would plants encroach again over the 

 country from the region to the south, 

 where its influence had been less severe. 

 What precise causes influenced most this 

 gradual northward march of aquatic and 

 terrestrial plants cannot now be deter- 

 mined, but probably they were such as 

 affect the distribution of plants at the 

 present day. The plants no doubt 

 followed the lines of least resistance and 

 greatest traction, not only in their geo- 

 graphical advance, but also in their 

 adaptations of structure and function to 

 the varying environments. These lines 

 must necessarily be ramified and in- 

 volved, perhaps to an insoluble degree ; 

 yet on them are the secrets of plant 

 geography to be discovered, on the basis 

 of physiological anatomy and plant 

 psychology. 



By such methods a most interesting 

 inquiry would be, What is the equi- 

 librium that has been attained between 

 the forces of resistance and traction that has caused certain ; have exhibited a tendency, as some do even now, to take 

 species to arrive at, and remain in, restricted areas? This on the aquatic habit, that mode of living being more 

 is a subject bristling with difiicult chemical and physical agreeable to their requirements. Some plants form them- 

 , complications, combined with the various influences result- selves into dense associations consisting of one species only, 

 ing from the never-ceasing action and reaction, not only which spread over considerable areas, and not only pre- 

 between the different members and associations of the vent others from growing amongst them, but year by 



NO. 21 18, VOL. 83] 



I- IG. 4 



An example of a small and shallow lowland loch having an abundant vegetation— Ottersioi 

 Loch, showing a portion o* a large association of Polygoniun amphibitim on the water in x.\\^ 

 foreground, and the bank below the trees covered with marsh pUnts, such as Carex rostrata 

 C. panicnlata, Phalaris arundinacea, &c. 



