THE ALG^E 93 



although occasionally a red pigment (Haematochrome) is present. 

 Such forms, however, may usually have pure green chromatophores 

 as well. 



Nucleus. A nucleus is always present in the cells, and there may 

 be more than one. 



The greater number of Green Algae are fresh-water organisms, or 

 may grow upon damp earth, trunks of trees, or other places where 

 a sufficient amount of water is present for their needs. Some of 

 them are marine, and others grow associated with other plants. 

 Thus the so-called "gonidia" of many Lichens, are Green Algae, 

 which may grow quite independently. Less commonly they occur 

 within the tissues of the higher plants. Chlorochytnum Lemnce 

 is a unicellular green Alga which lives within the intercellular 

 spaces of the little floating plant, Lemna trisulca. 



Plant-body. The simplest of the Chlorophyceae are unicellular, 

 but they are more commonly cell-rows, either simple or branching. 

 A smaller number (e.g. Ulva, Coleochaete) have a flat thallus. They 

 show very little external differentiation, this being most marked 

 in the Stone worts, or Characeae, which also contain the largest 

 members of the class. 



Reproduction. In spite of their simple vegetative structure, there 

 is a good deal of variety shown in their reproductive parts. Cell- 

 division occurs much as in the higher plants. Where the cells are 

 multinucleate, division-walls may be formed without a correspond- 

 ing nuclear division, but in the uninucleate cells, the nucleus under- 

 goes mitosis, as in the cells of the higher plants. In unicellular 

 forms, of course, each cell-division results in the formation of new 

 individuals. 



In most forms special non-sexual reproductive bodies are developed. 

 The simplest of these are naked, ciliated cells (Zoospores, Swarrn- 

 spores), which are formed either singly, or several together, from 

 the mother-cell, and after a longer or shorter period of activity, 

 settle down and form a new plant. Where these cells are destitute 

 of cilia, and develop a cell-wall within the mother-cell, they are 

 known as " Aplanospores." Less frequently, as in the buds, or gem- 

 mas of some Characeae, these reproductive bodies are raulticellular. 



Sexual Reproduction. Most Chlorophyceae show a clearly marked 

 sexual reproduction. The sexual cells in the lowest forms are not 

 distinguishable from the vegetative ones ; indeed, in some unicellular 

 forms like the Desmids, the protoplasts of two ordinary individuals 

 unite to form the sexual spore. More commonly, however, special 

 sexual cells, or gametes, are produced. These may be entirely 

 similar (Planogametes), or they may be more or less perfectly differ- 

 entiated into male and female cells. The product of the united 

 gametes is known as the Zygote, and usually becomes a thick-walled 



