Cultures and occurrence in nature 115 



is added to the sea-water. This medium was found to give constantly satis- 

 factory results without the addition of any sterilized organic matter. 



The best method of obtaining a culture was to add one or two drops of plankton to 

 250 cc. of the sterilized medium, which was then poured into Petri dishes. These should 

 be left undisturbed and exposed to moderately bright diffuse light. The temperature 

 should be kept as constant as possible and at about 15 C. In a few days colonies of 

 different species of diatoms will be observed on the bottom of the Petri dishes. These 

 can be removed by means of a fine pipette and transferred to flasks containing fresh 

 culture medium. Successful persistent cultures of single species may in this way be 

 obtained, but a word of warning must be given to the enthusiast who imagines that 

 in this way he may reap a rich harvest of a beautiful diatom. On the contrary, 

 deformed and distorted individuals are the rule in the earlier stages of the culture, and 

 the wall is often so feebly silicified that the characteristic markings are not present. In 

 older cultures the true form of the species is to a great extent regained, but the frustules 

 are small. 



Mention has already been made of certain saprophytic forms of diatoms which have 

 been produced by cultivating normal brown forms in suitable media. Many diatoms can 

 be cultivated on nutrient gelatin and agar, and some have been definitely shown by 

 Richter ('03) to liquify gelatin and to dissolve grooves in the agar into which they sink. 

 Miquel found the yellow rays of light the most favourable and obtained marked results by 

 placing cultures under yellow glass. In the case of plankton-diatoms, however, Allen & 

 Nelson were unable to obtain satisfactory cultures under yellow light. 



Diatoms will thrive in water at freezing point, but on the whole they 

 cannot withstand much freezing. There is evidence to show that the vitality 

 of many species is destroyed about 15 C., although it is reasonable to 

 suppose that some of the arctic and antarctic diatoms, more especially 

 the freshwater ones, are able to survive a much lower temperature. The 

 optimum temperature for cultures has been found to be about 15 C., and 

 the maximum, which is rapidly fatal, lies between 35 and 40 C. 



OCCURRENCE AND DISTRIBUTION. Diatoms are amongst the commonest 

 of microscopic objects and they are ubiquitous in all kinds of damp and wet 

 situations. They occur in great quantity in both fresh and salt water, and 

 some species, chiefly of marine origin, occur habitually in brackish water. 

 There is experimental evidence to show that apart from the quantities of 

 available nutrient materials, the salinity of the water can be varied within 

 large limits without appreciable effect on marine diatoms. On the other 

 hand, a relatively small degree of salinity is fatal to the majority of fresh- 

 water diatoms. These facts largely account for the preponderance of marine 

 species in brackish situations, and the almost complete absence of species 

 common to both fresh waters and the sea. 



Marine diatoms are most abundant in cold latitudes, having a decided 

 preference for cold water. Freshwater diatoms are abundant all the world 

 over, but they are more numerous and show greater diversity in temperate 



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