192 ASEXUAL REPRODUCTION OF ULOTHRIX 



sporangia. The zoospores resemble the motile plants noted 

 above, but after a short motile period they return to the station- 

 ary condition and grow into new plants (Fig. iii, C). In this 

 way, the number of plants is rapidly multiplied. The point 

 must not be overlooked that this phase of the life history is the 

 same as that of Sphaerella, the difference appearing in the 

 shortening of the motile period and in the lengthening of the 

 non -motile period. During the resting stage the cells of Ulothrix 

 to be sure may divide in one plane forming a filament but re- 

 member also that non-motile cells of Ulothrix msiy divide, forming 

 a mass of cells that become detached or loosely associated — the 

 so-called palmelloid state — which behavior we have seen sug- 

 gested in the non-motile cells of Sphaerella. It should be stated 

 that the asexual method of reproduction may be looked upon 

 as a means of bringing about a rapid increase in the number 

 of individuals of a species while the conditions for growth are 

 favorable. When new conditions arise, such as changes of tem- 

 perature, food, etc., that would interfere with the normal growth 

 of the individuals then a modification of the contents of the cells 

 and its mode of division results. The cells of Ulothrix as a con- 

 sequence of such alterations divide into a larger number of motile 

 bodies which are consequently smaller than in the case of zoo- 

 spores (Fig. 112, A). These bodies may behave as gametes and 

 unite, forming gametospores. The cells in which gametes are 

 formed are called gametangia, sing, gametangium. Thus, Ulo- 

 thrix is controlled in the same manner as Chlamydomonas and 

 Hydrodictyon. It is interesting to note, however, that these 

 small bodies frequently behave as zoospores and develop into 

 small and weak plants, which fact accounts for the common asso- 

 ciation in this genus of puny and vigorous plants (Fig. iii, C, £). 

 These small bodies of Ulothrix represent a curious intermediate 

 condition between a zoospore and a gamete where the sexual 

 character is not strongly enough developed to overcome com- 

 pletely the zoospore character. Thus, we see again in Ulothrix 

 that definite environmental conditions caused motile bodies to be 

 produced that are lacking in some essential substance and conse- 

 quently do not have the energy for growth, or at least can only 



