20 FIRST GROUP.— THALLOPHYTES. 



common gelatinous envelope, in others this proceeding does not occm-, and each cell by 

 itself forms an auxospore. 



Besides the auxospores, which as has been stated do not pass through a period of 

 rest, many Diatoms have also resting-cells which are usually spoken of as craticular 

 states. The cells in this case form a double cell-wall, i. e. two new valves inside the old one. 



III. SCHIZOPHYTA'. 



This class comprehends two divisions : one of forms containing chlorophyll, 

 whose green colourin'^: matter is mixed with a blue which is soluble in water (the 

 Cyanophyceae), and one of forms without c'llorophyll (the Schizomycetes). The 

 former live chiefly in water or else in wet places, sometimes as pseudo- parasites ; the 

 latter are either true parasites, or they live on the moist surfaces of organic bodies, or 

 they are found in fluids containing organic substances in solution, from which they 

 derive their food, and which they decompose by causing putrefaction or fermentation. 



The structure of the Schizophytes is always very simple, and in the simplest 

 forms the cells are so small that they can as a rule be seen only under high mag- 

 nifying power ; the cell-wall and the cell-contents are often scarcely distinguishable 

 from one another in the very minute forms ; where this is possible, the contents are 

 seen to be a homogeneous substance sometimes sprinkled with small granules ; the 

 cell-wall has a tendency to deliquesce into a thin jelly, in which the cells lie scattered 

 about or disposed in order ; sometimes the cell-wall is only swollen up and then is 

 plainly seen to be stratified. No cell-nucleus is found either in the Cyanophyceae 

 or in the Schizomycetes. 



In the simplest forms the cells live isolated ; when a cell divides, the halves 

 grow to the size of the mother-cell, divide again, separate and live an isolated life 

 by themselves ^ In the more perfect forms the cells produced by division remain 

 united, and according to the growth and the corresponding cell-divisions which ensue, 

 either simple and often very slender rows of cells are formed, or thin plates, the cells 

 from the mode of their division lying in one plane, or lump-like aggregations of 

 cells from the growth and divisions of the cells taking place in all directions. It is 

 only in the most highly developed species that the multicellular bodies assume a 

 definite external form. 



As a general rule the Cyanophyceae are larger and their cell-structure more 

 perfect than in the Schizomycetes ; here already on the lowest stage of the vegetable 

 kingdom we see how a degradation of structure is usually connected with absence of 

 chlorophyll. 



The cells of a Schizophyte are usually exacdy alike; but in some Cyanophyceae 

 a few larger and diff"erently coloured cells, termed heterocysts, are interposed at 

 intervals between the other cells of a chain. 



In most cases there is no distinction of base and apex, and therefore no definite 

 direction of growth ; it is only in the most highly developed forms that a base and an 

 apex can be distinguished, and this is accompanied by a certain kind of ramification. 



' [Zopf, Zur Morphologic der Spaltpflanzen, Leipzig 1883. — Schmitz, Fr., Die Schizophyten oder 

 Spaltpflanzen (Act. Leopold. XIX. 1883).] 



^ Owing to this peculiarity the name of the group is derived from (Txi'^'w, I divide, and (pvrov, a 

 plant. 



