OSCILLATORIACEJE. 811 



find in most species of Ulva a provision for extending the plant 

 by the dispersion of " zoospores ;" for the endochrome (Fig. 105, 

 a) subdivides into numerous segments (as at b and c), which at first 

 are seen to lie in close contact within the cell that contains them, 

 then begin to exhibit a kind of restless motion, and at last pass 

 forth through an aperture in the cell-wall, acquire four or more 

 cilia (d), and swim freely through the water for some time. At 

 last, however, they come to rest, attach themselves to some fixed 

 point, and begin to grow into clusters or filaments (e), in the 

 manner already described. The walls of the cells which have 

 thus discharged their endochrome, remain as colorless spots on 

 the frond ; sometimes these are intermingled with the portions 

 still vegetating in the usual mode; but sometimes the whole 

 endochrome of one portion of the frond may thus escape in the 

 form of zoospores, thus leaving behind it nothing but a white 

 flaccid membrane. If the Microscopist who meets with a frond 

 of an Ulva in this condition should examine the line of separa- 

 tion between its green and its colored portion, he may not 

 improbably meet with cells in the very act of discharging their 

 zoospores, which " swarm" around their points of exit, very 

 much in the manner that Animalcules are often seen to do 

 around particular spots of the field of view, and which might 

 easily be taken for true Infusoria ; but on carrying his observa- 

 tions further, he would see that similar bodies are moving within 

 cells a little more remote from the dividing line, and that a little 

 further still, they are obviously but masses of endochrome in 

 the act of subdivision. 1 . Of the true Generative process in the 

 Ulvacege, nothing whatever is known ; and it is consequently 

 altogether uncertain whether it takes place by simple conjuga- 

 tion, or according to that more truly sexual method which will 

 be presently described. Here, again, therefore, is an unsolved 

 problem of the greatest physiological interest, which probably 

 requires nothing more for its solution, than patient and discrimi- 

 nating study. And the Author would point out, that it is by no 

 means unlikely that the generative process may not be performed 

 in the complete plant, but, as in the Ferns ( 219), in the early 

 product of the development of the zoospore. Although the 

 typical Ulvacece are marine, yet there are several fresh- water 

 species ; and there are some which can even vegetate on damp 

 surfaces, such as those of rocks or garden-walls kept moist by 

 the percolation of water. 



196. The Oscillatoriacece constitute another tribe of simple 

 Plants, of great interest to the Microscopist, on account both of 

 the extreme simplicity of their structure, and of the peculiar 

 animal-like movements which they exhibit. They are conti- 

 nuous tubular filaments, formed by the elongation of their pri- 



1 Such an observation the Author had the good fortune to make in the year 1842, 

 when the emission of zoospores from the Ulvaceae, although it had been described by 

 the Swedish algologist Agardh, had not been seen (he believes) by any British naturalist. 



