FRESHWATER ALG.E. 115 



Desmidiaceao) the least conspicuous, but by no means the least inter- 

 esting member of the family, by reason of the facihty which they afford 

 for studying the phenomena of indefinite increase by cell-division. 



The obscurity of the characters of these minute forms has led to a 

 reckless multiphcation of supposed genera and species ; indeed there is 

 Uttle doubt that some of these really rej^resent different stages in the 

 life of the same species or even early stages in the development of higher 

 cryptogams. 



The simplest, and at the same time the most widely distributed 

 plant in this order, probably the most universally distributed of all 

 plants, is CMorococcuin vulgare, the humble organism to which the 

 green colour of the bark of most trees, of old paUngs, gates, &c., is due. 

 It consists of myi-iads of minute spherical cells, from l-1500th to l-2000th 

 of an inch in diameter, [Plate I., Fig. 1,] in which the process of division 

 of each original cell into two, four, or eight secondary ones, is admirably 

 exhibited. This appears to be the only process by which this species is 

 multix^lied, but in that which we shall nest consider we shall trace the 

 addition of a further mode of increase which prevails, with some modifi- 

 cations, through many of the more highly organised families of Algse. This 

 plant, probably aUied to the Protococcus j;/«rirtZ/s of some writers, 

 appeared some time ago in a freshwater aquarium, and consists of 

 innumerable spherical cells, fi-om l-1250th to l-2000th of an inch in 

 diameter, the outer coating of which takes the characteristic blue tint of 

 cellulose when treated with iodine. The inner gi-anular contents are 

 sometimes bright red and at others green. Both forms exhibit the 

 phenomenon of cell-di\dsion, gi\'ing rise sometimes to a still or motion- 

 less progeny, and at others to active zoospox-es, which move through the 

 water by means of pairs of cilia, or by a single cilium. The relations of 

 these two forms are somewhat obscure, and we have not been able 

 altogether to reconcile our own observations with the statements to be 

 found in books upon this subject. The red cells, at any rate, are 

 capable of Ijing dormant in a dry state for long periods, the active 

 processes of subdivision being re-estabhshed when water is added. 

 Plate I., Fig. 2, represents some of the conditions of this plant. Protococcus 

 pluvialis has been elaborately investigated by F. Cohn, whose research is 

 translated in the Ray Society's volume for 1853. 



We also give sketches of two kindred species, belonging respectively 



to the genus Vrococcus, Hass., [Plate I., Fig. 3] and Protococcus [Plate I., 



Fig. 4.] From these simj)lest forms there is an easy transition to such 



genera as Tetraspora, in which numerous green cells are arranged in a 



continuous gelatinous frond, in groups which show very beautiftdly then* 



reijeated sub-di\'ision into two and again into four, whence the generic 



name. Tetraspora luhrica [Plate I., Fig. 5] is abundant in summer in little 



streamlets among boggy ground in Sutton Park,* and is at all times a 



* Sutton Park is a property of some 2,500 acres, held under an ancient Charter 

 by the Corporation of Sutton Coldfield, for the beneiit of the inhabitants. It com- 

 prises woods, moor land, large sheets of water, and clear streams, and, being only 

 seven miles from Birmingham by raU, is the " happy huuting-gi-ound " of the 

 Naturalists of the town and district. Many of its rarer plants and animals have 

 gradually disappeared before the vast numbers of visitors who now n'equcnt it, 

 before recent " improvements," and still more before the railway works which have 

 sorely mutilated the beauty of some of its most charming parts. 



