THE MORPHOLOGICAL COMPOSITION OF PLANTS. 21 



Interpreting the facts in this manner, we may recognize 

 that large amount of truth which the cell-doctrine contains, 

 without committing ourselves to the errors involved by a 

 sweeping assertion of it. We are enabled to understand how 

 it happens that organic structures are usually cellular in their 

 composition, at the same time that they are not universally 

 so. We are shown that while we may properly continue to 

 regard the cell as the morphological unit, we must constantly 

 bear in mind that it is such only in a qualified sense. 



§ 181. These aggregates of the lowest order, each formed 

 of physiological units united into a group that is structurally 

 single and cannot be divided without destruction of its 

 individuality, may, as above implied, exist as independent 

 organisms. The assumption to which we are committed by 

 the hypothesis of evolution, that such so-called uni-cellular 

 plants were at first the only kinds of plants, is in harmony 

 with the fact that habitats not occupied by plants of higher 

 orders, commonly contain these protophytes in great abund- 

 ance and great variety. The various species of Pleurococ- 

 cacecs, of Desmidiacece, and Diatomacece, supply examples of 

 morphological units living and propagating separately, under 

 numerous modifications of form and structure. Figures 1, 2, 

 and 3, represent a few of the commonest types. 



Mostly, simple plants are too small to be individually 



a stage of development in which the nucleus has not yet been evolved, though 

 the chemical substance ' nuclein ' has been formed in some of them." 



Perhaps it will be most correct to say that, excluding the minute, non- 

 nucleated organisms, all the higher organisms — Metazoa and Metaphyta — are 

 composed throughout of cells, or of tissues originally cellular, or of materials 

 which have in the course of development been derived from cells. It must, 

 however, be borne in mind that, according to sundry leading biologists, cells 

 in the strict sense are not the immediate products either of the primitive 

 fissions or of subsequent fissions ; but that the multiplying so-called cells are 

 nucleated masses of protoplasm which remain connected by strands of proto- 

 plasm, and which acquire limiting membranes by a secondary process. So 

 that, in the view of Mr. Adam Sedgwick and others, the substance of an 

 organism is in fact a continuous mass of vacuolated protoplasm. 



