FOR FORMS AND (iUALlTlES. 43 



49. 



The divisions of the Nerves of the leaves also belong to this 

 department. 



Nerves, in general, are those visible continuations of the 

 leaf-stalk, or of the point of insertion, whici take place through- 

 out the length of the leaf The lateral branches of the nerves 

 are called Veins, which thus never run parallel with the axis, 

 but always form an angle with it. 



When the nerve is divided at the base, we name the leaf 

 according to the number of the nerves, three-nerved, five- 

 nerved (tri-nervium, quinque-nervium) ; but when the side- 

 branches of the principal nerves do not spring directly from 

 the base, but arise first a little above it, so that they have 

 some of the substance of the leaf under them, the leaf is then 

 said to be triple-nerved, quintnple-nerved (trrpli- qumtupH- 

 nervium). 



The veins and nerves often anastomose, or they are united 

 by side-branches {Vena anastomosantes, Anastornoses vena^ 

 runi). In the Ferns this is particularly observed. 



The Nerves are seen to have leafy processes (nervi lamel- 

 lati), in Gymnostomum ovatum, (Tab. IV. Fig. 9.) ; in Po- 

 lytrichum laevigatum Wahlenb., angustatum Brid. tenuirostri 

 Hook., and in some other mosses. 



Tlie nerves are said to be excurrent {excurrentes), when 

 they go on to the apex. The opposite ai'e interrupted nerves 

 (jiervi ad medium, ad § evanidi). Some also employ the 

 phrase. Folia ruptinervia. 



As the leaf-stalk commonly passes into the central nerve, 

 and two other nerves place themselves on the sides of it, 

 three and five nerved leaves are therefore the most conmion. 

 Two-nerved leaves appear almost only in the mosses, and 

 most distinctly in Neckera affinis. Hook. (Muse. Exot. ii. 

 t. 122.) 



50. 



In the division of branches and of stalks, the forked-shape 

 is the most common (rami, peduncidi dichotomi). In this 



