CELLS IN LOWER PLANTS. 93 



water of ponds, dissolved to a slight extent,* but quite sufficient 

 for the needs of Spirogyra and other algal plants. 



A filament grows in length only by the division of its terminal 

 cells, and this division involves, as a rule, the mitotic division of 

 the nucleus (see Chajj. viii.) ; but by suitably altering the com- 

 position of the medium in which the filaments are growing, one 

 observer has succeeded in changing the type of nuclear division 

 in Spirogyra from the " mitotic " into the " amitotic " form, in 

 the latter of which the nucleus divides en masse. This experi- 

 ment is one of great interest, as it shows that adaptation to 

 altered conditions may take place even in these low forms of 

 vegetable life. 



Conjugation in Spirogijra will be considered later under 

 "Reproduction" (Chap. ix.). 



h. Vaucheria. — In this plant, which is also a filamentous Alga 

 growing under conditions similar to those which obtain in the 

 case of Spirogyra, the filament is composed of one long tubular 

 cell, and not of a colony of separate cells joined end to end. 



Preparations of the fresh living filament may be first examined. 

 On mounting a filament, including its free-growing end, in water, 

 the following features will be noticed under a medium power of 

 the microscope : — 



i. A delicate eell-wall forming the outer boundary of the filament. 



ii. A narrow layer of eytoplasm lining the inner surface of the cell- 

 wall ; in this layer are to be seen (see Fig. 73, 1) — 



iii. Large numbers of small oval chloroplastS, and 



iv. Numerous small nuclei. These are usually only to be detected by 

 first fixing a filament in weak Fleraming's solution, washing, and staining 

 with carmine or hamatoxylin, with or without jjreliminary treatment of 

 the cell with alcohol to extract the chlorophyll. The nuclei lie close to- 

 the cell- wall, and arise by repeated divisions of pre-existing nuclei. 



The above structure — viz., peripheral protoplasm — in which lie 

 numerous nuclei, determines Vaucheria to be a coenocyte, a term 

 which denotes that a large number of " potential " cell-units are 

 present, enclosed by a common cell-wall (c/. Cells of fungal 

 hyphaj). 



V. Oil-globules are to be seen in the central space enclosed by the 

 protoplasm (central vacuole). Oil is manufactured by the ohloroplasts 

 of Vaucheria in the place of StaPCh (or sugar). On the addition of iodine 

 solution the nuclei turn a brownish colour, but no starch-granules show up. 



* The dissolved CO2 is present in the form of CO2 . HjO, or carbonic 

 acid. 



