164 



BOTANY 



PART I 



stem, and root. Similarly, the Red Seaweed, Hydrolapathum (Fig. 9), 

 although composed almost entirely of uniform cells, bears in its 

 external segmentation a resemblance to one of the most highly 

 organised plants. The internal differentiation of this Alga has only 

 advanced so far that the outer cells containing the red chloroplasts 

 form an assimilating tissue of isodiametrical cells, Avhile the internal 

 colourless and more elongated cells function as a conducting tissue. 

 The relatively highest degree of internal development 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 



Fio. 170. — Surface view and transverse section of the thallus of Marchantia polyinorphu. In A, an 

 air-poro, as seen from above ; in C, as seen in cross-section, (x 240.) 



grow in thickness through the continuous division of the cells of one 

 or more of the outer cell layers. A kind of cortical tissue is formed 

 as a result of this growth which exhibits concentric layers ; the 

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

 medulla. This secondary tissue even gives rise to sieve-tubes. In 

 the larger Fungi internal differentiation is the result of the more or 

 less intimate union of the intertwining hyjjhae. In extreme cases the 

 hyphae forming the body of the Fungus may be so closely Avoven 

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

 matous tissue (Figs. 109, 110), in which, on the subsequent thickening 

 of the cell walls, the pits in adjoining hyphae coincide. In the 

 fructifications of many of the Hymenomycetes and Gasteromycetes, 

 some of the longer and more swollen hyphae contain a homogeneous or 

 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, but even in them 



