24 W. H. GILBURT OX THE STRUCTURE AND 



or are the molecules themselves prepared on the surface of the 

 protoplasm and afterwards inserted ? In all probability the former 

 is the case, as otherwise the difficulty, which is now very great, in 

 accounting for some facts, would be greatly increased ; for instance, 

 the beautiful sculpturing which we find on the external surface of 

 many pollen grains is produced by forces acting from within, and is 

 due to growth by intussusception ; and while we cannot under- 

 stand how it is that these patterns should be always present and so 

 constant in design by any theory of growth, yet to suppose that 

 the molecules are prepared on the surface of the protoplasm and 

 then forced through would certainly not help, but rather otherwise. 

 It should not be supposed, however, that the presence of cellulose is 

 necessary to its further production in cells which are invariably 

 clothed with it. Strasburger has shown that the protoplasm of 

 the sac of Vaucheria may be made to contract and draw itself away 

 from the wall, when it will immediately commence to produce a new 

 one, free from the original ; and this experiment he repeated 

 successfully with the same part of the same sac. Another and 

 perhaps better example of the production of cellulose by the proto- 

 plasm apart from any already existing is seen in the formation of 

 the new wall in a dividing cell, the whole division plate being laid 

 down at once before any separation can be perceived in the proto- 

 plasm. We may therefore say, that while chemical affinity of 

 some kind plays a part in the production of these molecules, it is 

 and must be the result of physiological action. 



We come now to a consideration of the more important consti- 

 tuents of the cell, those parts in which life inheres, and upon the 

 presence and activity of which all growth and increase depends— 

 the protoplasm and the nucleus. We are all, doubtless, more or 

 less familiar with the appearance presented by a growing cell under 

 the microscope — say in a section not far removed from the apex of 

 a stem, or in a fragment of a very young leaf. The thin cell-wall 

 is apparent, and we find it either wholly or in part occupied with 

 a transparent semi-fluid substance, in which are imbedded a large 

 number of minute granules, which appear dark or bright according 

 as we look at them in focus or otherwise. In the midst of this 

 substance we observe a globose or ovoid body more highly refrac- 

 tive and much denser than the general contents of the cell. 



Protoplasm is sometimes spoken of as a fluid, but under no con- 

 ditions can such a term be strictly applied to it. A fluid always 



