DIATOMS— CONGER 327 



formed, lying back to back as in plate 4. Each new shell, together 

 with one of the original parent shells, and the protoplast lying 

 within forms thus a new cell. Subsequently, the old girdle slips 

 off and the two new cells appear as new individuals to repeat 

 the process. In this common and well-known method of vegetative 

 reproduction, the new shells being formed within, one of them is 

 in general necessarily smaller each time division takes place. This 

 process continued indefinitely would result in some of the cells be- 

 coming gradually smaller to the point of extinction were it not for 

 the intervention of another type of reproduction peculiar to the 

 diatoms, known as auxosporial reproduction, in which rejuvenation 

 of the size of the cell is achieved. 



In this process, in various ways in the different species, the proto- 

 plast expands, or two cells unite and the protoplast resulting from 

 this union expands, and the old shells are thrown off entirely. The 

 large gelatinous structure thus produced is known as an auxospore, 

 which after a time forms upon its outer membranous wall two 

 entirely new shells of larger size, and a new individual of large 

 size is produced. This innovation is necessitated by the rigid and 

 inflexible nature of the silica shells. 



The unique method of producing new shells within the old in the 

 ordinary process of vegetative reproduction assures that the offspring 

 will be, in outline at least, an exact copy of the parent. 



The girdle which plays so important a part in the structure, growth, 

 and reproduction of the cell, presents a number of nice adaptations 

 for its particular function. It must be such a structure as will hold 

 the two shells firmly together and prevent collapse of the cell at 

 such a point of frailty and yet permit of ready division of the cell at 

 the midregion for purposes of reproduction of the species. The 

 girdle provides satisfactorily for this, surrounding the cell at this 

 point, clasping the shells and holding them tightly together in posi- 

 tion, and yet it is capable of being thrown off at time of reproduction 

 (and without much loss of material) to allow division of the cell and 

 subsequent pushing apart of the two newly formed cells. In pro- 

 viding this arrangement, nature has in many cases designed the 

 girdle not as a complete hoop or ring, which would be inflexible, but 

 ]-ather as a band open on one side to permit it to be thrown off with 

 ease at the proper time. The same principle has been used by man 

 for the expansible rings formerly used to hold some types of auto- 

 mobile tires in place on the wheel, for the little expansible rings that 

 hold lenses in their mounts, and for the ring clips that hold some 

 types of fruit jar covers in place. ^Vlien this girdle band consists, 

 as is the case in many species of diatoms, not of one ring alone but of 

 several or numerous adjacent and interlocking ones interpolated from 



