18 



uo opportunity for growtli for our plant, which does not possess the ability of the 

 algæ to fix themselves ou stony ground. Along the open west coast of Jutland 

 the bottom is therefore nearlji devoid of Zostera; but wherever there is some shelter 

 we immediately find the grass wrack again, e. g. betweeu Fanø and the mainlaud 

 as well as beyond the political boundaries of Denmark along tlie island-rich west 

 coast of Schleswig. We may however seek for piaces with richer growths in our 

 own waters witliin the Skaw, and we find these already in Aalbæk Bay and further 

 on wherever the conditions are suitable for tlie plant. 



We come now to a third condition for the growth of the grass- wrack, that is, 

 the light. All green plauts (also the blue-green, the brown and the red) require 

 light to be able to thrive. The light is as a matter of faet a necessary source of 

 power to them, because by its means and by means of their own green pigment 

 they produce new stuffs for their growth from the carbonic acid of the air or water. 

 In this way they develop the organic substances from the inorganic and this 

 wonderful ability is the fundamental basis for all life on earth. The difference 

 between the distribution of plant life on the land and in the sea arises from the 

 dependence of the piants on light. While the land is covered with piants, the 

 low-Iyiug parts as well as the high, except where perpetual snow covers the ground 

 or the scarcity of water hinders the growth of the piants, all plant life in the sea 

 (save that of the bacteria) ceases at the depth of about 200 meters. The sea-bed 

 lying deeper tlian this is only inhabited by animals and bacteria; the reasou for 

 this is that the hght eau only penetrate dowuwards to a certain depth and below 

 this an impenetrable darkness reigns, only lighted up now and then by the Hash 

 from various marine animals with light-producing organs. The different components 

 of white light penetrate to different degrees down into the water, tlie amount of 

 refraction and absorption being different; the green, blue and ultrapurple comjio- 

 nents go furthest do\\;n and in connection with this we find that the redcoloured 

 piants, which just absorb these components, are to be found deepest down. On 

 the other hånd, the brown and especially the green piants cannot grow so deep 

 down, from which we may conclude that the eel-grass cannot grow at any great 

 depth. The more definite question: how far down can it grow, cannot be auswered 

 from this side, it must be answered from direct investigations. The result of my 

 observations in Danish waters is that the eelgrass goes much deeper down at 

 some piaces than at others, and that this is connected with the clearness (trans- 

 parency) of the water. At the time of year, in summer, when 1 made my observa- 

 tions, the water has as a rule its lowest transparency, as it then contains the most 

 plankton. But this rule does not hold in all cases, and besides, other factors are 

 present here; thus, the water near the coast is alwaj'S less transparent during or 

 after a storm or fall of rain than it is on calm days (comp. the observation from 

 Fæmø Sound on next page). To obtain however some standard for the transparency 

 of the water, a standard which, if not absolute, could yet give comparable data, I 

 employed a disc painted white, 20 by 15 cm. It was let down into the water fixed 

 to the lead-line. It proved that under fairly (juiet conditions the disc could be 

 seen nearly at tlie same depth as that to which the Zostera reaches, as the 

 following observations show: 



Lim Fjord, east of Livø brickfield ('"j-, lUUl). Soft ground. No Zostera 



