THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. IV. 



Boston, February, 1883. 



No. 2. 



Unicellular Algje. 



When we examine those simple 

 forms of life which consist of single, 

 isolated cells, we are at once met 

 with the query whether they are ma- 

 ture living organisms, or only transi- 

 tory stages in the life-history of more 

 complex forms. The answer to this 

 question is only to be given after long 

 and uninterrupted observation. We 

 have only to recall the method pur- 

 sued by Drs. Dallinger and Drysdale, 

 in their studies of the life-history of cer- 

 tain monads, which required constant 

 watching of single forms for days and 

 nights, to form some idea of the try- 

 ing nature of such investigations. If 

 it were, indeed, necessary to study all 

 the unicellular forms of life in this 

 way, we might well despair of ever 

 knowing much more about their differ- 

 ent .stages than we do now. But, for- 

 tunately, such continuous watching is 

 not always necessary. In many cases 

 it is so, and especially in the study 

 of the lower forms of animal life, which 

 usually pass most of their life in a 

 motile condition. Among the simplest 

 plants, however, the motile stage is 

 seldom of long duration. While there 

 are algae which pass most of their life 

 in a free-swimming stage like Volvox, 

 and its allies Pandorina, Gonium, Chla- 

 mido?nonas, with Oscillaria and cer- 

 tain diatoms, yet the vegetative life of 

 the algffi is usually passed in a state of 

 rest. Generally the motile condition 

 ser\'es the purpose of propagation. 



Among the phaenogamous plants, 

 various beautiful arrangements are 

 found calculated to assist in the dis- 

 semination of the seeds through the 

 air. There is the pappus of the dan- 



delion and thistle, the winged seed of 

 Cafalpa, the coma of the cotton-seed, 

 all of which are well calculated to 

 spread the seeds far and wide. For 

 plants living in water there must 

 be some other means of disseminating 

 the spores, or germs, and we find that 

 most of the algae and submerged 

 fungi produce free-swimming cells, 

 commonly termed swarm-spores be- 

 cause of their swarming out of the 

 cells in greenish, or colorless, cloud- 

 like masses, also known as zoospores 

 because of their animal-like move- 

 ment. 



Swarm-spores are spherical or oval 

 masses of protoplasm, or, perhaps, of 

 protoplasm including other cell-con- 

 tents, for they are formed directly 

 from the living matter of cells which, 

 at a certain stage of growth, contracts 

 into one or more balls within each 

 cell, and finally escapes by the rup- 

 ture of the cell-wall. Each swarm- 

 spore is provided with one or more 

 whip-like appendages, by means of 

 which it swims through the water. 

 The method of propagation by swarm- 

 spores is well illustrated by the very 

 common filamentous alga Chcztophora, 

 which is represented in fig. 6, a, show- 

 ing the swarm-spores escaping from 

 the apical cells of one of the branches. 



After a short time the swarm-spores 

 come to rest, and then they may 

 either grow directly into new plants, 

 or, becoming spherical and losing their 

 cilia, they may multiply for many gen- 

 erations, by a process of cell-division, 

 before again extending into filaments 

 like the plant which produced them. 



It is this stage of growth which is 

 so puzzling to the student of the algas, 

 and it may be desirable to consider it 



