- 4 6- 



are not clearly distinct, as hyemale and robustum, for instance ; 

 no description of one can be given which is not true of some form 

 of the other. 



DISTRIBUTION IN SPACE. 



It is remarkable that a genus of so few species should have so 

 general a distribution ; but more remarkable that the individual 

 species should have so wide a range. America seems to be the 

 home of Equisetum. Of twenty-five species given by Milde, 

 twenty-one are found in our hemisphere, and of these, nine are 

 peculiar to it, the remaining twelve having a cosmopolitan range 

 in the northern hemisphere, mostly north of 40 N. Lat. ; and two, 

 arvense and variegatum, were found by Greeley in Northern 

 Greenland. Arvense appears again in South Africa; only two 

 other species, ramosissimum and Telmateia, are found on that 

 continent, the latter but rarely. Europe has twelve species, Asia 

 thirteen, the United States fourteen, Mexico and South America 

 ten, and Australia one. 



GENERAL CHARACTERISTICS. 



The structure of Equisetum is on a single plan, the internode. 

 From the tip of the rhizome to the top of the stem there is no vari- 

 ation, save in degree. So similar are many species that it is only 

 by resort to the microscope that we are enabled, through examin- 

 ation of the interior structure, to recognize them. They are leaf- 

 less, jointed, perennial plants, often branchless and never forked. 

 The joints are regularly grooved and surmounted by a sheath, 

 bearing on its upper border as many teeth (or a few, more or less,) 

 as there are grooves and ridges on the internode. These teeth 

 are analogous to leaves, and are often spoken of as such. 



The epidermis of the stem is covered with a silicious coat, 

 which gives the plants characteristic harshness to the touch, and 

 wins for them the name of " scouring-rushes." This silex is vari- 

 ously set in granules, rosettes, rings, ridges, teeth, and several 

 other ways in the different species, being of considerable diagnos- 

 tic value, especially in the Hippochsetae, where most prominent. 

 The tissue of the stem may be destroyed by burning or by acids, 

 leaving the glass skeleton behind, which is then a beautiful mi- 

 croscopical object. 



The number and size of the ridges and grooves also enter 

 into the diagnosis, the former being angled, rounded or again 

 grooved, the latter bearing the stomata, the arrangement and 

 structure of which is a very important factor. 



