sect, i MORPHOLOGY 14fi 



The Phylogeny of the Internal Structure 



The phylogenetic differentiation in the internal structure of a plant 

 does not altogether coincide with the progress of its external segmenta- 

 tion. Even unicellular plants in the group of Siphoneous Algae 

 may exhibit a high degree of external differentiation ; thus the uni- 

 cellular Alga, Caulerpa (Fig. 250), has developed appendages having 

 outwardly the form of leaf, stem, and root. Similarly, the red sea- 

 weed, Hydrolapathum (Fig. 9), although composed almost wholly of one 

 form of cells, bears in its external segmentation a striking resemblance 

 to one of the most highly organised plants. The internal differentia- 

 tion of this Alga has only advanced so far, that the outer cells contain- 

 ing the red chloroplasts form an assimilating tissue of isodiametrical 

 cells, while the internal colourless and more elongated cells function as 

 a conducting tissue. The relatively highest degree of internal develop- 

 ment found in the Algae is attained by the Laminariae. In their 

 stem-like axis, which may have a considerable thickness, the external 

 tissues frequently contain canals filled with mucilaginous matter ; 

 while internally are found rows of cells resembling sieve-tubes. The 

 axes themselves grow in thickness through the continuous division of 

 the cells of an outer cell layer. A kind of cortical tissue is formed as 

 a result of this growth which exhibits concentric layers, and of which 

 the innermost cells gradually elongate and pass over into the so- 

 called medulla. In the Fungi internal differentiation is the result of the 

 more or less intimate union of the intertwining hyphse. In extreme 

 cases the hyph» forming the body of the Fungus may be so closely 

 woven together as to give, in a cross-section, the impression of a 

 parenchymatous tissue (Figs. 95-98), in which, by the subsequent 

 thickening of the cell walls, the pits in adjoining hyphaa are brought 

 into contact. In the fructifications of many of the Hymenomyeetes and 

 Gasteromycetes, some of the longer and more swollen hyphae contain a 

 cloudy, highly refractive, and, in some instances, coloured substance, and 

 appear, accordingly, to serve as a special tissue for' the purpose of 

 conduction. A marked advance in the differentiation into different 

 tissue systems is first apparent in the Bryophytes, and even in them 

 the formation of an epidermis distinct from the fundamental tissue is 

 exceptional. In the thallus of the Marchantieae of the Hepaticae, and 

 at the base of the spore capsules of the Bryineae, among the Mosses, 

 the external layer of cells becomes more or less sharply defined from 

 the underlying tissues. In the Marchantieae (Fig. 158) this outer layer 

 is pierced by openings which have been termed breathing-pores, but these 

 have a different origin from the stomata of higher plants. They are 

 rather, as Leitgeb has shown, openings into cavities, which have arisen 

 through the overarching of certain portions of the surface by other 

 more rapidly growing portions. In the Bryineae, on the other hand, 



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