20 



little lower tliau tluvt earlier given for our waters. Tims, C. G. Joh. Petersen 

 gives for tlie Kattegat 6 — 7 fathoms') and for the Little Belt 4 — 5 eveu 6 fathoms-). 

 J. Reinke^) states that he i'ound grass-wrack in the western part of the Baltic, 

 »vereinzelt bis zu 17 Meter Tiefe« on sand bottom, and E. Warruiug (Dansk 

 Plantevækst I, Strandvegetation, p. 185) says that »even as much as lU — 14 m. is 

 given for the Kattegat and the Little Belt«. I think however, I may suggest that 

 these reports rest upon the inaccuracy of the soundings; most of them come from 

 observations made for zoologicai or fisheries purposes, and for these oue fathom 

 more or less is quite immaterial. In the Kattegat (the place where all consider 

 that the grass-wrack goes deepest down in our waters) I have made particularly 

 careful observations to fix the limits and have come to the couclusion, that the 

 6 fathom curve is uever reached nor surpassed by growing Zostera; see 

 for the rest the specified data later (observation material, p. 47). 



We have statements from other countries that the Zostera vegetation grows 

 at greater depths. Thus, Karl Techet^) states for Trieste Golf that it grows 

 at a depth of 17 — 18 meters, and it is very probable that it grows deeper down 

 in the clear water of the Mediterranean than in our own less transparent waters. 



J. Reinke (1. c.) remarks that the grass-wrack goes deeper down on a 

 sandy than on a muddy bottom and this agrees with my own experience. The 

 explanation is most likely simply this, that the water over sandy bottom is clearer 

 (more transparent) than over muddy soil, where fairly large quantities of mud 

 particles are always being stirred up into the water. 



The nature of the bottom soil is an important factor in many regards 

 for the Zostera's growth. It has already been mentioned that the plant must have 

 loose (not rocky) soil for the roots to fix themselves on, but the varying cousistency 

 of the loose bottom is also of importance and aft'ects the growth and appearance 

 of our plant. The extreme kinds of bottom soil in which the grass-wrack grows 

 are on the one band the firm sand, sometimes covered with stones and on the 

 other the soft, muddy bottom of the quiet bays and fjords. It is fouud on 

 both sorts of ground, but the plant we find on the firm, sandy bottom is certainly 

 in many ways dift'erent from what I call the Mud-Zostera. We may examine into 

 this a little more closely. 



In the open waters where the motion of the waves is felt a good way 

 down, the bottom is firm and sandy to a considerable depth; there is no rest to 

 permit of the formation of the soft kind of soil generally cailed mud, which is 

 characterized by its abundance of organic matters (decayed pieces of piants and animals) 

 as well as by its fine clay particles. If we choose the Kattegat as a typical example 

 of an open water — and in this connection it must be counted as one, because 

 the occurrence of the grass-wrack depends on a certain even if but small amount 



') C. G. Job. Petersen: Det videnskabelige Udbytte af Kanonbaaden Hauchs Togter, 

 1893, p. 437, and Beretning IX fra den danslee Biologiske Station, 1900, p. 32. 



') C. G. Job. Petersen, Beretning lU fra den danske Biologiske Station 1893, p. 28. 



") J. Beinke, Algentlora der wostliehen Ostsee deutsehen Anteils, 1889, p. 12. 



■*) Ueber die Marine Vegetation des Triester Golfes. — Abhandl. der k. k. zool.-botan. 

 Gesellsch. in Wien, III. 3. 1906. p. 17. 



