24 



FRESH-WATER ALG.E OP THE UNITED STATES. 



stratcd the existence of anything indicating sexuality in the nostocs proper, or 

 shown any body at all worthy to be looked upon as a spore. 



Their ordinary method of reproduction is simply a slight modification of that of 

 growth. If a fragment of an actively growing nostoc is placed under the micro- 

 scope, the filaments of it are seen to be irregular and distorted, thicker in one 

 place .than another, the cells misshapen, and sometimes apparently lumped and 

 fused together. The formation of new filaments is taking place in such cases by 

 the simultaneous growth and longitudinal segmentation of the cells of the old, and 

 this may occur through the whole or in only a portion of the length of the latter. 

 (PL 2, fig. 10.) 



The filament of a nostoc is, in other words, capable of a double growth or de- 

 velopment, the result in one instance being increase in its length, in the other the 

 production of a new form like itself. The first of these is brought about by a 

 transverse division of the cells, so that out of each single cell two are formed, 

 placed end to end, each daughter-cell at first only half the size of their parent, but 

 soon attaining to its full stature. In the other case great increase in the size of 

 the cell occurs almost consentaneously with a longitudinal or lateral segmentation, 

 the cell dividing in the direction of its length, instead of transversely, so as to form 

 two cells lying side by side instead of end to end. The misshapen filaments alluded 

 to simply represent different stages of this change, which goes on until two perfect 

 filaments lie side by side, to be finally more or less widely separated by the jelly 

 which they secrete around themselves. 



This process of growth continues until the plant has arrived at its mature size, 

 when it ceases. During this time the inner portion of the frond has been be- 

 coming more and more liquid, and finally the outer epidermis bursts and the 

 thoroughly softened inner portion is discharged. In this way, innumerable filaments 

 are set free, which are endowed with a power of motion similar to, but much less 

 active than, the gliding of the oscillatoria, by means of which they are diffused in 

 the water. Scattered in this way, carried hither and thither by currents, each 

 minute thread, fixing itself to some object, at last becomes the centre from which 

 a new plant is formed in a manner similar to that already described. 



In the second division of the NostocJiacece, the jelly is always much less firm 

 than in the true nostocs, and is not condensed in the outer portions. The fronds 

 are therefore soft, almost diffluent, and entirely shapeless. The filaments them- 

 selves also differ from those of the true nostocs. There are no fixed differences 

 in the vegetable cells or heterocysts, however, although the former are apt to be- 

 come more cylindrical and the filament consequently less moniliform. It is espe- 

 cially in the possession of distinct reproductive sporangial cells that the differences 

 are to be found. These are much larger than the ordinary cells, from which, in 

 their first appearance, they are not distinguishable ; but, when the frond has attained 

 a certain age, the spore-cells begin to enlarge both in diameter and length, and 

 finally assume a form and size apparently fixed within narrow bounds for each 

 species, and surround themselves with distinct, often quite thick coats. It is very 

 possible that the production of new individuals may take place by a detachment of 

 portions of the frond and subsequent growth, as described in the Nostocs proper, 



