GROWTH AND CELL-DIVISION 



39 



The same takes place in Vaucheria and Mncor, and that these organisms 

 possess a latent power of inducing cell-division is shown during the 

 formation of sporangia, and also by the fact that under special conditions 

 Mucor assumes a yeast-like form, in which a tendency to divide is shown 

 while the cells are still exceedingly minute l . 



In this last case the change is induced by the external conditions, 

 which, however, merely act by modifying the internal disposition of the 

 cell. The same takes place when the external conditions induce or 

 prevent the production of sporangia and consequent cell-division in 

 Saprolcgnia or Vaucheria. Indeed various external agencies, such as 

 light, contact, gravity, or moisture, may influence not only the shape 

 of the plant but also the size of its component cells. Not only somatic 

 but also embryonic cells may be affected in this way, as is shown by the 

 changes in the size of a bacterium or of a Spirogyra cell according to 

 the cultural conditions 2 . We shall also see that cell-division may be 

 suppressed in plants in which it normally follows nuclear division, 

 although the latter continues. 



Between the extremes of size represented by a bacterium with a maximum 

 diameter of o-ooi mm., and by Caulerpa in which it may exceed 300 mm., all 

 grades exist. The isodiametric cells of the primary meristem vary from 0-005 

 to 0-024 mm - m diameter, and it is only as regards their longitudinal axes 

 that the cambium-cells exceed these numbers 3 . In the cells of permanent 

 tissues the diameter lies usually between 0-02 and 0-09 mm. ; although certain 

 cells, such as bast and wood-fibres, laticiferous cells, &c., may attain a considerable 

 length. The small size of the component cells renders possible a greater 

 complexity of tissue-differentiation, and it is easy to see the advantage of 

 using cells of approximately similar size for the construction of both large and 

 small organs. Indeed, if the cells in a particular organ, or in the entir 

 plant, were enlarged 100 to 1,000 times, its continued existence would become 

 impossible without a pronounced and far-reaching change of structure. 



The size of the protoplast cannot fall below a certain limit, although it is 

 not certain whether the minimal size is ever reached. To reduce the cell of 

 a Phanerogam to the minute dimensions of a bacterium would necessitate 



1 Klebs, Bedingnngen d. Fortpflanzung bei Algen u. Pilzen, 1896, p. 524. The older literature 

 is given by Klebs. A segmentation into cells can be caused by artificially dividing a filament of 

 Vaucheria into pieces, each forming new division-walls at its ends. 



2 Migula, Ueber d. Einfluss stark verdiinnter Saurelosung auf Algenzellen, 1888, p. 17. Cf. also 

 Klebs, Arbeit, d. Bot. Inst. in Tubingen, 1888, Bd. II, p. 537. 



3 Sachs, Flora, 1893, p. 49; Amelung, ibid., p. 176; Strasburger, Hist. Beitrage, 1893, Heft 

 5, p. 117. According to Nageli (Theorie d. Abstammnngslehre, 1884), a large lime-tree is composed 

 of about 2,000 billion cells. [A tree 25 metres high and i metre diameter might have an approximate 

 total bulk of 50 million cubic cms. in summer time, including the roots and leaves. Taking the 

 average diameter of the cells as 0-004 cm., and their average length as 0-05 cm., the bulk of each 

 would be 0-0000025 cub. cm., which gives a total of 20,000,000,000,000 in the entire tree. On the 

 dimensions and volume of bacteria see Fischer's Bacteria, Clar. Press, 1900, p. 4.] 



