52 KEY. G. HENSLOW — HOMOLOGY AND 



the apex elongates, and the internode stretches by interstitial growth 

 throughout its whole length. 



Conversely, stems can resemble roots in developing adventitious 

 roots ; and in absorbing moisture throughout their whole surface, 

 as long as it is only covered by a delicate epidermis and but a 

 thin cuticular layer. 



Normal instances of roots arising from stems occur in all 

 monocotyledons. 



As abnormal instances, — Exogens when decorticated often 

 produce them from the living edges of the part; all "cuts" and 

 " slips" are thus made to form independent plants by the facility 

 with which they will strike root. Adventitious buds are endogenous, 

 as are all new rootlets. 



Roots and stems can, both alike, be modified as reservoirs of 

 nourishment ; thus are tuberous roots of Ranunculus Ficaria, Orchis, 

 Pceonia, Dahlia, etc. Stem-structures having this function are 

 seen in rhizomes, corms, tuhers, etc. Lastly botli stems (I'adicle) 

 and root (axial, primary, or tap) may be together concerned in the 

 act, as in biennial roots of turnip, carrot, etc. 



Some plants have no roots when the stem supplies their functions 

 of fixing the plant to its site, as in Epipogon Gmelini, Corallorhiza 

 innata, and some Bromeliads, as well as the cryi)togamous bog-moss 

 Sphagnum, and Psilophyton. Utricularia has also no roots but is 

 simply suspended in water. In some of these cases the stems 

 would seem to act as absorbents of nutriment, as in the above- 

 mentioned orchids; so too similar (root?) structures in Neottia 

 probably absorb organic matter as saprophytes. 



An interchange of functions can be brought about artificially. 

 Thus Duhamel caused the bougbs of a willow to be bent down to 

 the ground, buried, and to strike root. When these were well rooted, 

 he extracted the original roots and elevated them, so that the tree 

 became inverted ; the roots now threw out leafy shoots. 



The functions of stems, as compared with roots, may be classified 

 as follows : — (i) to fix the plant to its site when it has no roots ; 

 (ii) to absorb water through its epidermis, and other food as in 

 saprophytes ; or to act parasitically, as in Viscum, etc. ; (iii) as 

 climbing instruments in stem-twiners ; (iv) as reservoirs of nutri- 

 ment, as in tubers, etc. ; (v) as propagating instruments, as by 

 bulbs, tubers, the aerial corms of Ranunculus Ficnria, and the 

 swollen disarticulated bratwhes of Vttis gongyloides ; (vi) as extend- 

 ing the individual, as by creeping stems of grasses, rhizomes, etc. 



II. Appendicular Structijkes — Vegetative, 



1. The Leaf -scale Regions. Leaf-scales. — The first modifications 

 of phyllome to be considered are Leaf- and Bud-scales. These 

 occur in two regions, ( 1 ) on underground stems, as well as on the 

 basal parts of aerial stem- axes, and (2) as aerial buds. 



Leaf-scales of the first kind appear to have only two origins. 

 They are either homologous with the basal or petiolar portion of the 



