158 



LECTURES 



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Fig. 23. 



Figs 21 and 22. izatioQ of fuDctioii as development progresses. 

 The longitudinal system is first formed by modifi- 

 cation of the cellular, and then this is again differ- 

 entiated into the two forms of woody and vascular 

 tissue. Now, in the pine family, this last differ- 

 entiation has not taken place. So far as its tissues 

 are concerned, therefore^ this family should rank 

 helow all other flowering plants. 



Let us next examine the different classes of 

 plants with respect to the venation of their leaves. 

 With respect to their venation the leaves of 

 plants are divided into three distinct kinds, viz : 

 the reticulated, or netted veined, the parallel 

 veined, and the dicholomously veined. In the 

 first the veins branch and again run together, 

 forming an inextricable net-work, (Fig. 23, a.) 

 In the second the veins run parallel from one end 

 of the leaf to the other, connected only by slender 

 transverse bars, so that the leaf may be torn into 



parallel rib- 

 bons. (Fig. 

 23, &.) In the 

 third the veins 

 branch, but do 

 not run to- 

 gether again. 

 (Fig. 23, c and 

 d.) The first 

 is characteris- 

 tic of the Dico- 

 tyledons ; the 

 second of the* 

 3Ionocotyle- 

 dons; and the 



third of the Ferns — perhaps of the vascular Cryptogams generally. 

 The leaves of cellular Cryptogams are veinless. In this enume- 

 ration it will be observed I have not mentioned the Conifers. To 

 which class, then, do the leaves of the pine family belong? Undoubt- 

 edly to the third. This fact cannot be easily demonstrated upon 

 leaves of ordinary pines, for their cylindrical leaves show no veins, 

 or, if visible, they seem to be parallel. But there are a few broad- 

 leaved Conifers, and these always show the dichotomous branch- 

 ing of the veins in the most unmistakable manner. In the 

 Salisburia, for instance, we have as beautiful an instance of this 

 mode of branching as can be found among the Ferns. The leaves 

 of this Conifer are about two or three inches broad, the shape and 

 venation very similar to that represented in Fig. 23, c, but much 

 more beautii'ul. This close alliance in the venation of the leaves 

 between the pines and the ferns is another evidence of the low 

 position of the former among flov.'ering plants. Thus it appears that 

 this remarkable family of plants is allied to the highest Fhcenogams 



