336 Conjugate 



breadth of the chloroplasts or the filament may be in a position such that 

 only the edge of the chloroplasts can be seen (fig. 210 J5). 



The action of light in causing the rotation of the plate-like chloroplasts of Mougeotia 

 has been known for a long time. In diffused daylight they place themselves at right-angles 

 to the direction of the incident rays, but the edge of the plate is directed towards strong 

 sunlight. It was shown by Lewis ('98) that the chloroplast occupies on an average 

 30 minutes to rotate through 90. 



The nucleus is in most cases elliptical in its broadest outline and much 

 compressed, with one flattened side closely adpressed to the chloroplast 

 (fig. 210 B n). It contains a large nucleolus. 



Vegetative multiplication occurs frequently in nearly all the species of 

 Mougeotia by the dissociation of the filaments into their constituent cells, 

 each of which may form a new filament by subsequent rapid cell-division. 

 The dismemberment of the filament is primarily caused by a split in the 

 transverse separating walls. This split, which becomes lenticular in shape 

 and is occupied by mucilaginous material probably derived from the middle 

 lamella, gradually gets larger until it has the form of a thick double-convex 

 lens. At this stage increase in the turgidity of the cells causes the firmer, 

 previously concave, terminal walls to become convex, the area of attachment 

 gradually becoming less until the cells fall apart as the mucilage disappears. 



Of the known genera of the Mesocarpese only Mougeotia is of general 

 world- wide occurrence, and in this genus the conjugation is of peculiar 

 interest. The exact facts of this conjugation have been known for a long 

 time, although their interpretation has been controversial. Conjugation 

 occurs almost always between the cells of the different filaments which are 

 lying side by side. Each cell puts out a protuberance on the side towards 

 the other filament and this meets with a similar protuberance from one of the 

 opposite cells. The ends of the protuberances come into close contact, the 

 separating walls are absorbed, and an open tube is formed placing the two 

 conjugating cells in communication with each other. This channel of com- 

 munication is known as the conjugation-tube. Each conjugating cell is a 

 gametangium, and, during the development of the protuberances and their 

 ultimate fusion, the greater part of the protoplast of each gametangium, 

 including the nucleus and chloroplast, passes into the conjugation-tube. The 

 gametes, which are thus formed from only part of the protoplast of the 

 gametangium, fuse in the conjugation-tube to form a zygospore. Nuclear 

 fusion does not occur until some time afterwards. The subsequent happenings 

 are entirely peculiar to the genus Mougeotia. Instead of at once surrounding 

 itself with a new wall this zygospore becomes cut off from the gametangia by 

 partition- walls (two to four in number according to the details of conjugation), 

 so that at first the wall of the zygospore consists of a part or the whole of the 

 walls of the conjugation-tube together with newly formed partition-walls. 



