50 THE CELL 



in surface can only take place by means of intussusception; there are 

 a number of facts which suggest, that where membranes grow in this 

 way, they do so more or less independently of the protoplasm. 21 



C. THE DIMENSIONS OF CELLS. 22 



Among both plants and animals " the size of the individual 

 is a quantity which varies within very wide limits, whereas the 

 dimensions of the structural units or cells are restricted within 

 a much narrower range of variation." The quotation is from Sachs, 

 who was the first botanist to draw attention to this apparently 

 unimportant but in reality highly significant circumstance. The 

 tallest trees, such as the two species of Seqvoia and certain Eucalypti, 

 may attain a height of over 100 metres (330 feet), while 

 the smallest Bacteria are scarcely one thousandth of a millimetre 

 ( 1 ,oi = -o-tfoo inch) in length. The linear dimensions of the individual 

 thus vary roughly between 1/u. and 1 00,00 0,0 00/x. In the case of 

 the cells of which the individual is composed, the extreme limits of 

 size lie much closer together. It appears from Amelung's measure- 

 ments that the diameter of the more or less isodiametric parenchy- 

 matous cells which have the largest share in the composition of 

 the plant-body, generally amounts to between "015 and '066 mm. 

 Sachs' statement to the effect that " the transverse diameter of an 

 adult parenchymatous cell is always measurable in hundredths of 

 a millimetre " (or in other words always lies between *01 and TO mm.) 

 is a less exact but perhaps a more convenient generalisation. The 

 cells composing the pulp of fleshy fruits and the pith of certain 

 stems (e.g. Sambucus nigra and Impaticns glandulifera), may attain 

 a diameter varying between T3 mm. and 1 mm., and may thus 

 be readily distinguishable with the naked eye; but it. is quite ex- 

 ceptional for parenchymatous cells to exceed the above-mentioned 

 average dimensions. Elongated prosenchyrnatous elements often reach 

 a much greater size. Thus bast-fibres are commonly between 1 and 

 2 mm. in length, but occasionally attain a length of 10 mm. (Flax and 

 Hemp) or even of 50-200 mm. (certain Urticaceae). The diameter 

 of the smallest cells, on the other hand, rarely falls below "01 mm. in 

 multicellular plants, which may in fact be said to consist of structural 

 units, the linear dimensions of which lie, generally speaking, between 

 10/x and 10,000/U. It is obvious, however, that such numerical 

 data only give a very rough idea of the actual facts. 



The principal outcome of Amelung's measurements is the demon- 

 stration of the fact, that when homologous organs of one and the 

 same plant for instance, different foliage leaves vary in size, the 

 dimensions of the component cells exhibit little or no variation In 



