SEA WEEDS 369 



special leaflets near the tips of the frond. D. sinuosa is a less 

 common weed, with a disc-like root and an oblong, cleft and 

 toothed frond, and tetraspores in leaflets growing from its margin. 

 Another species D. hypoglossum is characterised by the leaflets 

 of the midrib bearing still smaller leaflets in the same manner. 



We have already referred (p. 366) to a sea weed commonly 

 known as the Dock-leaved Delesseria, the scientific name of which 

 is Maugeria (Delesseria) sanguinea. This plant was once included 

 in the present order, but has been removed on account of the dif- 

 ferent structure of its fruit. 



Our next order is the interesting one containing the coral-like 

 weeds, some of which are so common and so conspicuous in the 

 rock pools. The order is known as the Corallinacece, and all its 

 species secrete carbonate of lime, which hides their vegetable struc- 

 ture and gives them more the appearance of stony corals. 



The typical genus (Corallina) includes two weeds with jointed 

 pinnate fronds, and spore-conceptacles at the tips of the branches 

 with a terminal pore. 



These and the allied sea weeds are very unlike plants in their 

 general nature, their stony covering of carbonate of lime hiding all 

 traces of the delicate cellular structure so characteristic of the various 

 forms of vegetable life, and especially those of aquatic or marine 

 habit. If, however, the weed is put into dilute hydrochloric 

 (muriatic) acid the calcareous matter will be completely dissolved 

 in a minute or two, with evolution of bubbles of carbonic acid gas ; 

 and if a portion of the frond be then examined in a drop of water 

 under the microscope, the cellular structure referred to will be seen 

 as well as in any other weed. Another characteristic of the plant, 

 or rather of the carbonate of lime which it secretes, is its property 

 of becoming intensely luminous when held in a very hot flame. 

 Thus if a tuft of coralline be held in the flame of a Bunsen 

 burner, it will glow so brilliantly as to remind us of the lime light. 

 Further, if we examine the plant in its natural state, we find that 

 the carbonate of lime is not secreted uniformly in all parts, but 

 that the nodes of the jointed frond are free from the stony deposit, 

 and are therefore flexible. 



Our commonest species G. officinalis maybe found in almost 

 every rock pool between the tide-marks, growing on rocks, shells, 

 and other weeds. The joints of the stem and branches are 

 cylindrical or somewhat wedge-shaped, while those of the branch- 

 lets are linear ; and the colour varies from a dark purple to white, 



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