144 



of trees, gate posts, and various other situations suitable to their 

 existence. Some of the species were formerly thought to belong to 

 the animal Kingdom owing to their possession of the power of move- 

 ment, but their position as vegetables is now too well defined to admit 

 oi further controversy. 



These plants are devoid of roots, and they are determined 

 principally by the form of the cells of which they are composed. 

 Some species live in social relationship with Fungi under the name of 

 Lichens and also with some of the lower forms of animal life. Kerner 

 and Oliver p. 113 Vol. 1. state that. "Saprophytic marine and fresh- 

 water alga- are able to absorb the products of decay in the water 

 around by means of a superficial layer of cells. 



The Besmidiiiceo?, and some of the various filamentous alga' accu- 

 mulate secretions of calcareous matter, and the Diatomaceai secrete 

 silicious material in building up their cells. 



Alga> abound in the Arctic as well as in temperate and tropical 

 climates, and are most commonly found in still pools or slow moving 

 waters; but some species are to be found in rapid running streams 

 and even in waterfalls. 



The green scum or slime which appears in reservoirs, and 

 aqueducts, is chiefly composed of these organisms, and the presence of 

 such material is often thought by the superficial observer to show 

 contamination ; but as a matter of fact the (/rent majority of the species 

 will not c.n'st in contaminated water, and are readily killed by the 

 addition oi' minute quantities of Ammonia or Copper in liquid form. 



Instead therefore of being direct evidence of contamination, Alga? 

 afford strong evidence that the water in which they grow is uncom- 

 taminated. 



The Diatomacese are found in both salt and fresh water, while the 

 Desmidiacese are only found in fresh water, and a large number of 

 species are to be found in Trinidad waters. Wolle in his work on the 

 Desmids says: "' We are under no small obligation to them for aiding 

 to keep the atmosphere in a wholesome condition since they absorb 

 carbonic acid largely, and exhale Large quantities of oxygen." 



Some species under cultivation in the Botanical Laboratory at 

 St. Clair, prove very clearly thai Wolle's statement is correct, for not 

 only can the bubbles of oxygen gas he seen to develop and rise to the 

 surface hut it also becomes apparent how these plants actually make 

 use of the gas bubbles to effed contact with the air, as the organisms 

 are gradually brought to the surface by aid of the rising bubbles. 

 Observations have shown thai large numbers of these plants rise to 

 the surface during daylight and again descend to the bottom during 

 the night hours. Indeed they form an excellent lecture room example 

 of the evolution and dispersion of oxygen. They can he mad. to 



evolve oxygen much more freely, if a -mall quantity of acid is added 

 to the water in which they are growing. 



A.lgae are generally presenl in waters as mixed communities of 



species; and in our present collection some ten or twelve species have 

 been noted, among which the genua Cosmarium is the most oumen us ; 

 in company with one or two filamentous forms and a few oi the 



smaller species of Diatomacea'. 



