48 PRIMARY RIBS [CH. 



The vascular bundles which come off as main strands 

 from the petiole are termed primary, and these bear 

 secondary strands, which in their turn branch into ter- 

 tiary, &c., and so on, until the bundles of higher orders 

 generally the fourth or fifth are too fine to follow. As a 

 rule we do not enumerate any of higher order than the 

 tertiaries, except that the blind ends of the last ramifica- 

 tions are called terminals. 



In by far the majority of plants such as we have to 

 consider, there is but one primary rib, the midrib, running 

 vertically and continuously from base to apex of the 

 lamina, and becoming thinner and thinner as it passes to 

 the tip of the leaf; as it runs up it gives off secondary 

 ribs from either side, of slightly narrower calibre than 

 itself, which pass towards the margins, and in their turn 

 give off tertiaries, &c., which break up into a less and 

 less conspicuous meshwork. Such venation is termed 

 pinnate, because the secondaries come off from the midrib 

 somewhat as do the parts of a feather from the quill 

 (Fig. 14). 



In some broad leaves, however, the median primary 

 or midrib is accompanied at its origin by two, four or 

 more nearly equally strong ribs which divaricate from 

 it, on either side, in their further course in the lamina, 

 and since these lateral radiating ribs are usually quite as 

 conspicuous up the median portion of each lateral lobe of 

 the leaf to each of which, in fact, one of them acts the 

 part of a midrib to its lobe in the same manner as the 

 central one does to the median lobe, or to the whole leaf 

 they may be regarded as primaries. Such venation, with 

 several radiating primaries all springing from one point, 

 is termed palmate (Fig. 15). 



In particular cases, where these lateral primaries of a 

 palmate venation need particularising, owing to differences 



