LICHENS. 689 



they die their shelly covering remains, and we find the shells or cases in all 

 earthy formations. These diatomaceye have been raised from the beds of 

 oceans, and Atlantic soundings have revealed their presence, as mud, when 

 examined, proves to be these remains of vegetable shells. Thus the infi- 

 nitely little in the animal and vegetable worlds meet at the bottom of the 

 sea, as well as on dry land. 



There are marine and fresh-water algae the former familiar to us as 

 seaweeds which possess air-bladders that children love to explode, and 

 which assist the algae to float. They attach themselves to rocks, generally 

 at the base ; the lovely colours of seaweeds are well known. They will be 

 recognized under the name of " tangle " (fucus), which, when burned, gives 

 kelp and barilla, which is full of iodide and sodium. The Sargasso Sea is 

 composed of miles of algae which live in the open ocean, The Carrageen or 

 Irish moss is very nutritious and useful in consumptive cases. Indeed, all 

 algae, if not absolutely useful, are certainly not deleterious. The " bladder- 

 wrack" was formerly useful for the production of soda. 



" The life-history of one of these uni-cellular plants in its most 

 simple form, can scarcely be better 

 exemplifLed than in the Palmogecea 

 macrococca, one of those humble forms of 

 vegetation which spreads itself as a green 

 slime over damp stones, walls, etc. When 

 this slime is examined with a micro- 

 scope, it is found to consist of a multi- 

 tude of green cells, each surrounded by 

 a gelatinous envelope ; the cell which 

 does not seem to have any distinct mem- 



. J Fig. 820. -Bladder wrack. 



branous wall is filled with granular 



particles of a green colour, and a 'nucleus' may sometimes be distinguished 

 through the midst of these. When treated with tincture of 

 iodine, however, the green contents of the cell are turned to 

 a brownish hue, and a dark-brown nucleus is distinctly 

 shown. Other cells are seen, which are considerably elon- 

 gated, some of them beginning to present a sort of hour- 

 glass contraction across the middle ; in these is commencing 

 that curious multiplication by duplicative subdivision which 

 Fig. 8 2 i.-Lichen. j s the mode in which increase nearly always takes place 



throughout the vegetable kingdom." * 



LICHENS are numerous, and may be found upon the bark of trees in 

 dry forms of grey and yellow growth, and on wails and old stones in our grave- 

 yards. On the hills we find them growing upon the granite, and it would 

 appear that they prefer stone to any other holding ground. The Arctic 



* Carpenter on the Microscope. 



44 



