PLATE XIII— COMMON BLADDER WRACK (Fucus vesiculosa) and TANGLE (Laminaria digitata). 



Fucus (Gr. phukos, sea-weed) and Laminaria (Lat lamina, a thin plate) may be taken as representatives of the brown-coloured sea- 

 weeds. They are common objects of the shore wherever rocks abound. 



In Fucus, the flat expansion or Thallus is dichotomously branched, and attached to the rocks by suckers, so that there is a super- 

 ficial resemblance to stem, roots, and leaves. But it is only superficial, since the whole plant is bathed with sea-water from which, and 

 not from the soil or air, every part withdraws its appropriate nourishment. The root-like portion consists of delicate hair-like branches 

 with thin cell-walls. It acts like a boy's sucker; it can be pressed very close to the rock, and the pressure of the water, just like the 

 air in the previous case, keeps the two together. 



The stem-like narrow portion, as well as the more expanded upper portion, is slimy all over, and this is due to the cell-walls of the 

 outer cells becoming mucilaginous. 



The air-bladders serving the purpose of floats contain various gases. 



The reproductive organs are borne by the swollen ends of branches and developed in little cavities known as Conceptacles. These 

 are seen by the naked eye as little elevations with openings, and have been formed by a pusning-in or indentation of the exterior. 

 Each dimple or Conceptacle contains Antheridia with Antherozoids or male organs, and Oogonia with Oospheres or female organs. 



The most common species are F vesiculosus (Lat. vesicula, a little bladder) with a midrib running along each part of the thallus, 

 and air-bladders arranged in a double row; F nodosus, with air-bladders arranged singly and no midrib; and F. serratus (Lat. serra, 

 a saw) destitute of air-bladders and margins toothed. 



Laminaria digitata (Lat. digitus, the finger) is so named because the expanded portion is split up like the fingers of the hand. It 

 has a root-like portion consisting of numerous branching stalks expanded at their attached end; a stem-like portion which is perennial, 

 and increases in thickness by concentric layers added year after year ; and the split-up leaf-like portion which is renewed every year. 



Multiplication takes place by Zoogonidia developed from the expanded portion. Sexual reproduction is as yet unknown. 



Fig. 1. Portion of plant, natural size. 



Thallus branching in a forked manner or dichotomously, with a well-marked midrib. 



Air-bladders occurring in a double series. 



Fertile branches swollen and studded over with little papillae. 



Fig. 2. Make a transverse section of a fertile branch, so as to get one of these little papillae in section which are called Con- 

 ceptacles. There is a confused mass of hairs, amongst which may be seen the male and female organs. The close- 

 set cells of the exterior are continued right round the Conceptacle, thus suggesting an infolding of the exterior and not 

 an interior cavity afterwards opening externally. 

 Antheridia, branching hairs. 

 Oogonia, swollen hairs. 

 Figs. 3 and 4. Take some of the yellow colouring matter from Conceptacle and mix with salt-water to see Antheridial hairs and 

 Oogonia clearly 



Antheridial hairs repeatedly branched, the ends of the branches swollen and filled with yellow granular matter. 

 When ripe the contents of these cells consist of Antherozoids each provided with two cilia whereby they move rapidly 

 about in the water. 



Oogonia are globular bodies, derived from a single cell and producing eight Oospheres. The protoplasm of 

 the surrounding hairs and stalk is broken up into threads, because of the numerous vacuoles formed, owing to the 

 cells getting too large for their contents, as in Spirogyra. 



Fig. 5. Oogonium discharging its contents. 



The wall of the Oogonium consists of two layers — an outer, inelastic, which splits, and an inner, extensible, 

 which stretches a deal before giving way. The Oospheres are discharged into the conceptacle, then into the sur- 

 rounding water. 



Fig. 6. The liberated Oospheres meet with Antherozoids which surround them, blend with them, and convert them into Oospores, 

 ready to germinate. 



Fig. 7. Germination of an Oospore. 



It first becomes pear-shaped, then divides into two, and the tapering end soon develops organs of attachment. 

 The upper end divides further and further in all the dimensions of space until the adult form is attained. 



Life History. — Fucus reproduces itself sexually by Antheridia and Oogonia, either produced together or on separate plants. The 

 Antherozoids of the Antheridia fertilise the Oospheres of the Oogonia after being set free, and each Oospore thus produced- 

 may develop a new plant. 



Histology. 



Fig. 8. Make a transverse section of the narrow stem-like portion, and examine in alcohol or glycerine under low power. 

 Cells close-set towards exterior, but arranged loosely in interior. 



Fig. 9. Stain transverse section with magenta, and examine under high power. 



Cells are round, oval, or elongated, and cell-walls very gelatinous. 

 Fig. 10. Cut across Tangle and examine— -first, with naked eye; second, a transverse section under low power. 

 (a.) Outer yellowish-brown portion, and inner colourless portion. 



(b.) Outer coloured portion, small and close-set cells. In old specimens there is a ring of oval slime-cavities pretty 

 near one another. Inner almost colourless portion of larger cells. 

 Fig. 11. Make a longitudinal section, and examine interior cells as in Fucus. 



The elongated cells are bounded by a firm inner membrane, and between this membrane, of two adjoining cells, 

 there is a gelatinous intercellular substance often arranged in layers. Short Pits occur here and there in the 

 membrane. 



