PELAGIC PLANT LIFE 321 



mass in the middle, and become enwrapped in a new thick wall 



of characteristic shape within the old cell-wa 

 carded as soon as the resting-spore 

 is completely developed (see Fig. 

 223). The spores have now ac- 

 quired an increased specific weight, 

 as compared with their original cell, 

 and sink down into deep water, 

 where they may be found months 

 after they have disappeared from 

 the surface-layers. The majority of 

 them, however, rest on the bottom 

 in shallow coastal waters, until con- 

 ditions of existence again occur 

 which induce them to make a fresh 

 start. 



The germination of the resting- 

 spores has not yet been described, 

 though Hensen states that Lohmann 

 has observed the first stages on 

 several occasions. It will be a great 

 advantage when we can follow their 

 development-history through all its 

 stages, and study the conditions of 

 existence that lead to germination. 

 Resting-spores are unknown in the 

 true oceanic species ; but, as already 

 stated, they are found in most of 

 the species belonging to coastal seas, 

 not aware of them till quite a short 



^hich 



Fig. 223. — Chain of Chmtoceras 

 constrictum, with three rest- 

 ing - spores and one normal 

 cell (the end - cell of the 



CHAIN) {*-r-). 



In some cases we were 

 time ago. It is only 

 recently that they have 

 been discovered in Lep- 

 tocylindrus danicus (see 

 Fig. 224), in which the 

 cylindrical cells are 

 broken across in the 

 Fig. 22af.—LEPTocYuxDRus danicus, WITH RESTING- process of spore-forma- 

 sFORE(i"/^). ^j^^^ g^ ^^^^ ^^^ spores 



are liberated, and in Chcetoceras pseudocrinitum, in which the 

 resting-spores originate in auxospores. 



So far as we are able to ascertain, the auxospores of pelagic 

 diatoms are always formed without any sexual act. There is, 

 however, another kind of organ, the so-called microspores, Microspores. 



Y 



