358 



DEPTHS OF THE OCEAN 



(Fig. 249), which had not been met with subsequently. The 

 whole structure of this diatom shows that it, too, is most 

 probably a neritic form, and it must therefore have a wider 

 distribution than was commonly supposed.^ 



As we neared the coast banks of Europe we found the 

 number of species growing distinctly less, though on the other 

 hand the quantity of the plankton increased. 



The plants of the sea like those of the land build up all the 

 organic substance which forms the chemical foundation of life. 

 If we wish to know clearly when and how and under what 



^^^' conditions vigorous production takes place, or what prevents 



the development of an exuberant plant-life, we must first 

 acquire the means of estimating the amount of vegetation in the 

 different parts of the sea. 



Hensen. Hcnsen was the first to take up this problem, the solution 



of which depends on three assumptions: (i) it is absolutely 

 essential to have apparatus that can capture all the organisms 

 living in a specified quantity of water, (2) the plankton must 

 be supposed to be uniformly distributed in the sea, so that the 

 catch represents a reasonably extensive area ; and (3) a scientific 

 examination of the catch must supply a really correct picture of 

 the amount of plants and their capacity of production. 



Hensen'snet. The apparatus employed by Hensen and his assistants 



consisted of extremely fine straining-cloth, with meshes 0.04 to 

 0.05 mm. in diameter. He made the mouth of his net small in 

 proportion to the filtering silk surface, to ensure as far as 

 possible the immediate filtering of all water that came in through 

 the opening, his object in this being to ascertain approximately 

 how much water was filtered, when the net was drawn through 

 the sea for a calculated distance. Experiments showed that in 



^ As illustrating a haul on this section I append a list of the species found in the closing net 

 at Station 8i (lat. 48° 2' N., long. 39° 55' W. ), from a depth of 50 metres to the surface : — 



Diatoms : Coscinodisciis excentrictis, Euodia c2ineifor»ns, Planktotiiella sol, Coscinosira 

 (Estrtipi, Thalassiosira subtilis, Corethron C7-iophilum, Rhizosolenia styliformis, R. shrubsolei, 

 R. fragillima, R. alata, R. semispina, Baderiastruni delicatulum, B. elongatum, Chatoceras 

 atlanticum, C. boreale, C. mediterraneiim, C. peruvianum, C. criophilum, C. decipiens, 

 C. contoftuiii, C. schiittii, C. curviseium, C. lacmiosum, C. furcellatum (a resting-spore), 

 Thalassiothrix longissitna, T. nitzscktoides, Nitzschia seriata. 



Peridinese : Ceradum lineatu7n, C. candelabruvi, C. pe,7itagonum , C. gravidum, C. fusus, 

 C. pennattim, C. tripos, C. azoricum, C. gibberum, C. plat y come, C. arcticiim, C. intermedium, 

 C. macroceros, Protoceraiium reticnlatum, Peridinium oceanicum, P. depressum, P. divergens, 

 P. conicum, P. ovatjim, P. tristylum,z.nA some others, Diplopsalis leiitictda, Pyrophactis horologium, 

 Goniodotna polyedricm/i, Gonyaulax polygram ma, Podolampas elegaiis, P. palmipcs, Oxytoxum 

 scolopax, O. diploconus, Ptychodiscus carinatus, Dinophysis acuta, D. schiittii, D. rotundata. 



Flagellates : Phceocystis poticheti. 



Silicoflagellates : Dictyocha fibtda. 



Chlorophycese : Halosphcera viridis. 



Cyanophycere : Trichodesmium thiebaulti. 



