SCALE-LEAVES, FOLIAGE-LEAVES, FLORAL-LEAVES. 629 



lating system, is already found in the stem. From this they extend through 

 the leaf-sheath and petiole to the base of the blade. This last is therefore in 

 a manner the entrance-gate for the strands, and as soon as they have passed it 

 f>. division takes place not unlike that of a stream which flows from a narrow 

 valley mto a plain, and there breaks up into numerous larger and smaller 

 branches; or they may perhaps be still better compared to an aqueduct whose 

 mam stream is inclosed and strengthened by masonry and embankments, but 

 which branch out, at the confines of the town which has to be supplied with 

 water, into several subordinate conduits which penetrate the difierent districts, 

 and then again break up into numerous smaller water-pipes leading to the 

 buildings and other places of consumption. 



We may distinguish two kinds of distribution in respect of the course of 

 these strands as they enter the leaf. In the one case there is only a single 

 thick strand, the primary strand, which distributes itself and breaks up inside 

 the narrow gate. In the other, three or more distinct main strands pass over, side 

 by side, into the blade, each following a separate course. As a rule, these are 

 connected by bridges and mter- networks. Thus we distinguish between leaf- 

 blades with a single main strand and those with several. 



Leaf -blades with one main strand may be sub -divided into two groups 

 according to the form and course of the lateral strands which arise from the 

 primary one. Either these lateral strands are all weaker than the main one, and 

 originate from it successively, at intervals, like the ribs of a spinal column, or 

 like the barbs on the axis of a feather, when we speak of a feather-like (or 

 pinnate) arrangement of the lateral strands (see figs. 149 i. 2. s, 4, s, e, 7, lo, is'). q^ ^j^g 

 lateral strands are almost as strong as the main one, arise from it directly at 

 the base of the blade, and run out from this point like rays towards the margin 

 of the lamina. This arrangement of the lateral strand is called radiating (see 

 figs. 149 «■ 9. 11. 12). 



When the lateral strands are arranged like a feather, it generally happens 

 that they are alike in the matter of strength, that they are distributed 

 symmetrically over the whole blade, and originating at fairly equal intervals 

 from the main strand, take, at least at first, a parallel course. More rarely 

 it happens that stronger and weaker lateral strands alternate, and that they 

 diverge from the primary one at unequal angles. In the Camphor Tree 

 {Laurus Camphora, fig. 149*), the Cinnamon Tree {Cinnamomum), and many 

 other plants related to the Bay Laurel, this peculiarity is found, viz. that two 

 lateral strands which proceed from the lower third of the main one are 

 stronger than the others, looking as though a three-pronged fork had been 

 inserted in the leaf. In the Wall-Pellitory (Parietaria), whose leaves show 

 a similar character, stronger and weaker lateral strands alternate, and, strangely 

 enough, the stronger spring from the main strand at an acute and the weaker 

 at a right angle. For the rest, the lateral strands with feather-like arrangement 

 may be distinguished as reticulate, looped, arched, and undivided. 



