40 



BOTANY 



FIG. 27. Ciliate cells. 



Spirillum undula. B, zoo- 

 spore of Cladophora sp. C, 

 spermatozoid of Equisetum 

 maximum. 



male reproductive cells both of plants and animals. Cilia are ex- 

 tremely delicate threads of protoplasm, 

 which are extensions of the outer hyalo- 

 plasm, or, in the case of Bacteria, of the 

 cell-membrane, which is evidently not 

 similar in composition to the protoplasm. 

 Sometimes there is a single stout cilium, 

 or flagellum, but more commonly there 

 are two or more. The movement of the 

 cilia is very active, and more or less 

 undulatory. Ciliary movement is only 

 possible in water, and is the method of 

 propulsion of all free-swimming cells 

 (Fig. 27). 



Where the protoplast is enclosed within 

 a cell-membrane, it cannot shift its posi- 

 tion beyond the confines of the cell ; 



nevertheless, active movements can often be seen within the proto- 

 plast, and careful study will reveal slower movements within most 



cells, resulting in a shifting of the position of different organs. The 



protoplast may, also, in some cases, escape from the cell, as in the 



formation of zoospores, and 



it then for a time resumes 



the power of locomotion by 



developing cilia. 



Movements within the Cell. 



An enclosed protoplast 



may show three types of 



movement. The first of 



these, " Rotation," occurs 



within the cells of a number 



of aquatic plants; e.g. the 



elongated cells of Chara (Fig. 



28) and Nitella, the leaf-cells 



of Vallisneria and Elodea. 



In these plants, the cyto- 

 plasm forms a thick layer 



lining the cell-wall, and 



surrounding a large central 



vacuole. The hyaloplasmic 



layer next the wall does not 



take part in the movement, 



and in Chara and Nitella the chloroplasts remain stationary; but 



in Vallisneria the chloroplasts are carried along with the rotating 



granular cytoplasm, which moves in a direction corresponding to 



FIG. 28. A, portion of a rhizoid of Chara 

 sp., showing the rotating cytoplasm ; the 

 arrow indicates the direction of the current 

 (X about 200). B, surface-view of a large 

 internodal cell from a leaf of the same 

 species, showing the arrangement of the 

 chloroplasts and the neutral line which 

 contains no chromatophores. 



