INTRODUCTION. 35 



their earliest period, viz. from the condition of zoospores, and 

 having examined their structure and modes of growth, we 

 shall next proceed to make a few remarks upon their dis- 

 tribution and vitality.* 



Not amongst the least beautiful of the many minute or- 

 ganizations, whose intimate structure the microscope, which 

 has so wonderfully extended of late our knowledge of the 

 natural world, has revealed to us, are the freshwater Alga, 

 and yet the majority of these constitute the rejected and 

 despised, by all but the true naturalist, scum and slime of our 

 still and soft waters ; but although many freshwater Algce 

 are, for an obvious and benevolent purpose, hereafter to be 

 mentioned, ordained by their Great Designer to be the tenants 

 of our impure and stagnant waters, there are other species 

 which are met with only in fresh and running streams, ad- 

 hering by one extremity to some object of attachment, the 

 other floating freely in the surrounding fluid medium in the 

 course of the stream, whose impetuosity and strength these 

 frail productions seem at first sight but ill able to withstand. 

 They find their protection, however, not less in the flexibility 

 than in the tenacity of their structure. This is the case 

 with the Lemanice, Lyngbya crispa, and with the beautiful 

 Conferva glomerata, which delight in the purest and most rapid 

 streams. The Ectosperma clavata of Vaucher, known by its 

 globular form and dark green shining appearance, is met 

 with only in the course of the waterfall or cataract, sus- 

 taining unharmed the whole force and weight of the foaming 

 waters which pour over it. The Batrachosperms, the most 

 elegant of all our freshwater Conferva, also usually dwell 

 in pure water, but are obliged, for the most part, from the 

 delicacy of their conformation, to confine themselves to such 

 streams and rivulets as are slow, and possess but little 

 strength ; while some Conferva, as many species of Zyg- 

 nemata, Tyndaridea and Mougeotia, are almost exclusively 

 confined to marshes, ditches, or shallow and extended pools, 



* For a highly interesting memoir upon the connection of the cells of 

 plants, by Hugo Mohl, see Annales des Sciences Naturelles, second series, 

 torn. viii. (Botanique). 



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