BACTERIA. FUNGI. 161 



cover both fresh and salt-water plants. In still inlets of the sea it is not rare to see 

 the larger sea-wracks with smaller' specimens clinging to them, whilst Florido;e are 

 fastened to the latter, and minute siliceous-coated diatoms to the Floridese. Even 

 in fresh water, e.g. in cold and rapid mountain streams, we find little tufts of 

 Chantransia or Batrachospermum developed as epiphytes upon the black-green 

 filaments of Lemanea, and on the former, again, Diatomaceae. One of these 

 Diatomaceae, which, from its resemblance to a scale insect, has received the name of 

 Cocconeis Pediculus, is especially conspicuous, and is often found by the score upon 

 the green filaments of Algae. Such a connection does, no doubt, suggest the idea 

 that the Cocconeis drains the green algal cells of nutriment; nevertheless, such an 

 assumption is not well founded, and if algas, beset by Cocconeis, derive injury at all 

 from their presence, it is chiefly owing to a restriction of their absorption of 

 nutrient substances from the surrounding water and to interference with their 

 respiration. 



The distinctive property of true parasites does not lie, therefore, in the habit of 

 growing upon other plants and animals, or even in the fact of killing their living 

 supports, but resides exclusively in the withdrawal of nutrient substances from the 

 living vegetable or animal bodies which they invest. 



The plants and animals attacked and drained of their juices by parasites are 

 called hosts. 



From the point of view of food absorption, true parasites may be classified in 

 three groups. The first group includes generally all microscopic forms which live 

 in the interior of human beings and animals, chiefly in the blood; the second 

 comprehends fungi possessing mycelia, which have the power of withdrawing by 

 the entire surface of their filamentous cells, or by clavate outgrowths of the same, 

 nutritive material from the tissues of the host invaded by them; and the 

 third group comprises flowering plants wherein the seedling, upon emerging from 

 the seed, penetrates into the host, by means of suction-roots or some other part 

 which subserves the function of a suction-root, so as to absorb juices from the 

 host. 



BACTERIA. FUNGI. 



In treating of parasites of the first group, we must, in the first place, refer to 

 several of the unwelcome visitors known by the name of Bacteria. They appear 

 to be invariably unicellular, sometimes spherical, sometimes shortly cylindrical or 

 rod-shaped; some are straight, and others curved in arcs or spirals; a few are non- 

 motile, whilst some are actively motile. The largest forms have a diameter of 

 -5-J-y mm.; the smallest do not measure more than o^o mm., and are reckoned 

 amongst the minutest organisms hitherto revealed by the aid of the best micro- 

 scopes. In liquids of suitable chemical composition and temperature, they multiply 

 with extraordinary rapidity, reproduction being effected by division. The rod- 

 shaped cells elongate somewhat and divide into two equal halves, each half, when 

 grown to a certain size, divides once more into two, and so on without limit. 



VOL. I. 11 



