BOTANICAL SUBJF.CTS. 139 



Le Maout and Decaisne ("System of Botany," 1873, p. 130) 

 write thus : " The anatomical structure of leaves is the same as 

 that of the stem ; they consist of a fibro-vascular bundle and 

 parenchyma ; this bundle, which is wholly formed before leaving 

 the stem, spreads into a blade, as it emerges, in sessile leaves, or 

 remains undivided for a certain distance before expanding, in 

 petiolate leaves. The nerves of the blade are formed of fibres and 

 vessels ; both it and the petiole are covered Avith a layer of epi- 

 dermis bearing stomata on every part, excepting the nerves and 



petiole The relative position of the elements of 



the libro-vascular bundle, which passes from the stem into the 

 Iciif-blade, may be compared to a flattened stem, the fibres and 

 vessels of which have been spread out, and thus allowed plenty 

 of room for the development of parenchyma between their 

 ramifications." 



On p. 131 they further say that, " Tlie leaf originates as a 

 small cellular tumour, which afterwards dilates into a blade, the 

 cells on the median line of which elongate and form fibres, 

 then, as in the stem, first tracheae, and lastly other vessels." 



Asa Gray (" Structural Bot.," p. 47) says in a note : " In phai-no- 

 gamous plants only the ramifications of axes should take the name 

 of branches, that is root and stem branches ; and the term mav 

 without confusion be extended to hairs, and all trichomes when 

 compound, but not to leaves and their modifications (I)." 



"With the light thrown by Bower and others on the nature of 

 the leaf, Asa Gray's statement is no longer tenable. Where is the 

 line to be drawn ? Leaves are undoubtedly branch systems, and 

 evidently a further development, in a different medium, of the 

 cladophyls of seaweeds. Their sub-divisions are as much branches 

 of the petiole and midril) as the root divisions are branches of the 

 root axis. 



Why the veins are in one plane, and not all round the midrib 

 as they generally are round the stem proper (but not alwavs, as in 

 the common yew) is becfiuse they inherit that mode of emero-ence 

 owing to the necessity of exposure of the parencln matous surface 

 to the light. To do this, the leaves must be horizontal and look 

 up\vards, although there are many exceptions to this rule, owiucr 

 to other disturbing circumstances. This necessity, being forced 



