14 



MORPHOLOGICAL DEVELOPMENT. 



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

 physiological units united into a group that is structurely 

 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 Protococcus, 

 of Desmidiacece, and Diatomacece-, supply examples of morpho- 

 logical units living and propagating separately, under nu- 

 merous 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 

 visible without the microscope. But, in some cases, these 

 vegetal aggregates of the first order, grow to appreciable 

 sizes. In the mycelium of some fungi, we have single cells 

 developed into long branched filaments, or ramified tubules, 

 that are of considerable lengths. An analogous structure 

 characterizes certain tribes oiAlgce, of which Codiitm adhcerens, 

 Fig. 4, may serve as an example. In Hydrogastrum, an- 

 other alga, Fig. 5, we have a structure which is described as 



