FUCUS VESICULOSUS. 241 



older parts of the thallus, especially from its edges, numerous 

 adventitious shoots can often arise. The growing points of the 

 thallus lie at the apices of the twigs, in cleft-like depressions, the 

 direction of which corresponds with the plane of the leaf-like 

 expansion, and which can be readily recognised with the lens. 

 Some of the apices may be seen to be double, from equal forking, 

 or dichotomy ; but quickly one branch surpasses the other, the 

 latter becomes pressed to one side, and appears as if laterally 

 developed. 



For anatomical investigation alcohol material is almost as well 

 suited as when fresh ; but fresh material can be sent for great 

 distances without injury, when packed in cases without water, 

 In what follows, therefore, we will concern ourselves preferably 

 with alcohol material, as this permits section-cutting much more 

 readily! In fresh material the swelling of the tissue between the 

 outer and inner layers of the thallus is so strong that it produces 

 great contortion of the sections. The outer layer is in positive, 

 the inner in negative tension ; that is, the outer layers are com- 

 pressed by the inner layers, and the latter stretched by the outer. 

 Therefore, in the sections, the outer layers lengthen and the inner 

 layers shorten, and the sections twist. Preparations made from 

 fresh material must be examined in sea-water, since they swell 

 strongly in fresh water ; alcohol material, on the other hand, we 

 lay a few days previously in a mixture of equal parts alcohol and 

 glycerine, and examine it in the same. 



We prepare surface sections, parallel to the surface of the 

 thallus, through the midrib and the wings (and, if we wish to see 

 only the primary tissue, at not too great a distance from the grow- 

 ing point), besides cross and longitudinal sections through different 

 regions. The entire surface of the frond shows, in surface sections, 

 rectangular to polygonal cells, which are arranged more or less 

 clearly in longitudinal rows, often displaced by supplementary 

 divisions. In cross and longitudinal sections these cells appear 

 prismatic, and elongated perpendicularly to the surface. They are 

 clearly distinguishable from the layer of tissue which lies next 

 below, and we will distinguish them as the outer or epidermoid 

 layer. The cells of this outer layer are closely tilled with olive- 

 coloured chromatophores, The chromatophores have the form 

 of rounded grains, or are become polygonal by lateral pressure, 

 and contain the characteristic colouring matter of the Fucaceae, 

 phaeophyll, or phycophaein, To the outer layer succeed ele- 



16 



