374 Vegetable Physiology. 



almost every case, a distinction between the absorbing organs 

 (roots), the digestive and respiratory organs (the leaves), and the 

 organ which at once serves to connect these together, and con- 

 stitutes the focus of development (the stem). Tliis distribution 

 of labour is accompanied, from the lowest forms in Avhich 

 it appears, upwards, by the coincident occurrence of a kind 

 of tissue absent in the lower group ; the fibrous and fibro- 

 vascular cords which, connected together and arranged in various 

 ways in the stems, form a bond of union between the organs, 

 and, in the more highly developed plants, constitute a skeleton 

 or framework to support the almost indefinite products of the 

 vital activity of the properly cellular tissues. 



The first group are called Thallophytes, from thallos, a Greek 

 word signifying a vegetating shoot, and phyton a plant. The 

 vegetative structure consists of a homogeneous mass, such as we 

 see in the fronds of Sea-weeds or the scaly patches of the Lichens ; 

 and as this vegetative mass or layer is exclusively composed of 

 cells comparatively little changed from their primary c(mditions, 

 these plants are sometimes distinguished as Cellular plants. The 

 cellular tissue does indeed exhibit very considerable diversities 

 and a considerable range in the degree of alteration from the 

 original form of a membranous sac, as is evident Avhen we com- 

 pare the simple confervoid filament with the larger Sea-weeds, 

 in which there is a distinction into cortical and medullary tis- 

 sues, evident both from the form and texture of the cells. But 

 the thallus never presents any trace of those specially metamor- 

 phosed and regularly arranged masses of elementary tissue which 

 constitute the fibro-vascular cords of the higher groups. 



A most important kind of gradation does, however, present 

 itself within the limits of the Tiiallophytes, dependent on 

 a matter which we have not yet touched, namely, the specializa- 

 tion of cells in reference to the reproductive functions. In the 

 very lowest forms, as in the fresh- water Algae, the same cells 

 form in the early part of their existence the organs of vegetation 

 and growth, and at a later period give up these functions and. 

 undertake the production of the spores, the germs of new indivi- 

 duals for the reproduction of the species. Step by step, in more 

 complex forms of Alga?, the reproductive functions become more 

 localised, at first in certain selected cells of the vegetative mass ; 

 afterwards the reproductive cells are found marked for their 

 special function from their very first origin ; and in the highest 

 forms, portions of the thallus are developed into -^ecxxWuv fruits 

 or receptacles, enclosing and protecting the reproductive cells. 

 These distinctions in the reproductive structures are of great 

 importance in the eyes of tlie botanist and of the physiologist ; 

 but their interest is almost exclusively scientific, and they bear 



