V] SECONDARIES 51 



apical subulate tooth, e.g. Quercus coccinea. In most 

 cases, also, it divides the lamina into two equal halves 

 i.e. it is strictly median but there are exceptions, e.g. 

 Lime, and especially some tropical plants. 



The secondary ribs offer diagnostic characters of some 

 importance in the angle of divergence from the primary 

 ribs. This angle may be small less than 45 but in 

 the majority of cases it approaches 55 on the average, 

 e.g. Cctfttanea : it is however occasionally a right angle, 

 and in rare cases is greater than a right angle with the 

 forward part of the midrib. Moreover the upper second- 

 aries often come off at a different usually more acute 

 angle than the lower. 



As with the primaries, so with the secondaries, we 

 have strong, weak and excessively fine strands in different 

 species. 



Their length is best expressed in terms of the breadth 

 of the leaf, or of the length of the midrib : it refers to 

 that part of the secondary which can be distinctly traced 

 from its origin to where it breaks up above into the 

 network. As regards their length, relative to each other, 

 the secondaries near the middle of the leaf are usually the 

 longest, those above rapidly and those below gradually 

 becoming shorter; but plenty of cases occur where the 

 upper secondaries are longer, and even where secondaries 

 exceed the midrib in length. 



Very useful characters are derived from the direction 

 and course of the secondaries. They may run quite 

 straight to the margins, as in the Hornbeam, Chestnut, 

 &c., or curve out in more or less divergent lines, as in 

 the Beech, or convergent, as in the Birch, or they may be 

 sinuous, as in Quercus coccinea. 



The rule is that they thin off gradually, but in some 

 cases the ends suddenly thin out to capillary veins. 



42 



