INTRODUCTION TO CRYPTOGAMIC BOTANY. 131 



upon Biatomacece almost exclusively. Dr. Hooker tells us 

 that he does not remember to have examined the contents of 

 the stomach of any Salpa between the latitudes of the N. 

 Tropic and 80° S. which did not contain the remains of Dia- 

 tomaceoe. Dictyocha aculeata was universally found in the 

 stomachs of those he opened when off Victoria-land. Tripoli 

 powder is largely used for polishing, in consequence of its being 

 composed of silex, and some species form valuable tests of the 

 power and clearness of microscopes. Though the greater part 

 of DiatomacecB do not readily enter into a state of decom- 

 position, the GymhellecB form an exception, probably from 

 comparative defect of silex. 



4. CONFERVACE^, Ag. 



Threads articulated, simple or branched, attached or more 

 rarely free ; mostly of an herbaceous green. Cells generally 

 longer than their diameter. Propagation by minute zoospores, 

 or metamorphosed joints. 



104. This is a vast tribe consisting of very numerous species, 

 which are in general of very simple structure, but according to 

 their mode of aggregation, or branching of the threads, exhi- 

 bit extremely varied aspects. They grow in various situations, 

 both in fresh and salt water : in thermal spriags, on the bare 

 soil, when sufficiently moist to afford nutriment, on exposed 

 rocks, on the leaves or fronds of aquatic plants, indeed, in 

 almost every situa,tion in which vegetable life can be sustained; 

 but we must not extend the term to those productions which 

 affect mineral solutions and liquids, impregnated to a greater 

 or less degree with organic matter, for they are mere states of 

 different moulds, which may occasionally be detected iu the 

 act of bearing normal fruit. Nor am I inclined to include 

 those nearly colourless species which grow parasitically on dead 

 or diseased animals, as I feel convinced, notwithstanding the 

 very curious phenomena they exhibit, that they are really states 

 of EungL If, however, they are to be considered as true Algse, 

 their place will be here, and I shall, therefore, describe the 

 peculiarities they exhibit under this head. 



105. The threads are either simple or branched, and contaia a 

 nearly colourless endochrome, as in moulds; and, indeed, it was 

 9 * 



