50 EEV. G. HENSLOW — HOMOLOGY AND 



The term leaf may be used in a wide sense for any phyllome or 

 appendage, not inclnding trichomes. Hence we may recognise 

 phyllomes as occup3ing (1) the leaf-scale and bud-scale regions, (2) 

 the true leaf region, (3) the bract* region, and (4) the floral region. 



Homology asserts, jf?r«?!, that root- and stem-axes are fundamentally 

 the same ; secondly, that all leaf-appemlages or phyllomes are 

 fundamentally the same ; and thirdly, that even caulomes and 

 phyllomes must be regarded as being fundamentally the same ; 

 though in each case their normal functions may be very different, 

 respectively. 



In comparing these several organs, we must first consider their 

 normal differences, and secondly look for their fundamental 

 agreements. 



I. AxiAx STRUCTirEES — Yegetative. 



The following comparisons may be made between root- and stem- 

 axes. Roots differ from stems, — 1, in their order of Iranching ; 

 2, in their anatomy ; 3, in their phyaiology ; and 4, in the absence 

 of buds and appendages, excepting epidermal root-hairs. 



1 . Branching of Roots. — The branching of stem-axes being mainly 

 determined by the position of the leaves, and these being more or 

 less arranged according to phyllotactical laws, the branching 

 becomes more or less symmetrical, or at least is originally so, 

 but the regularity is often marred by the arrest of buds and by 

 their displacement, etc. In roots, there is, except at first, no such 

 apparent regularity. As examples of certain regular distributions 

 the following may be noticed: the rootlets of Cruciferce, PapaveracecB, 

 Resedaceo', and Geraniacece are in two lines ; several Leguminosch in 

 three lines; Malvaceae, Euphorbiaceoi, Umhellifera, Lahiatce, and Ver- 

 henacece in four lines ; and Compositm and Solanece in five lines. The 

 arrangement in two and four lines is the commonest. This dis- 

 position is generally in accordance and in connexion with the 

 fibro-vascular bundles. 



2. Anatomy of roots. — This is in some respects very different 

 from tliat of stems. A young root consists of epidermis with or with- 

 out root-hairs and with no stomata. Beneath it is the cellular 

 cortical layer; then the cylinder of cells called the pi'otecting 

 sheath which includes the root-forming pericambium, and lastly 

 the central fibro-vascular column usually without a distinct medulla 

 or pith. The apex is protected by a special root-cap or pileorhiza, 

 wanting in Gymnosperms. Old woody roots as compared with 

 similar stems have mostly no pith, and the medullary rays are less 

 numerous and less developed. The wood is very irregular. There 

 is much interlacing of parts, causing the wood to be knotty. There 

 is a similar cambium layer. The cortex or cork is mostly thicker 

 (as in the thick cellular layer in herbaceous rhizomes). There is 

 less liber or none. 



* Bracts may be regarded as transitional organs between the vegetative and 

 reproductive. 



