62 



LECTURE V. 



Sometimes, however, the body of the bulb is composed of the subterranean basa] 

 portions of leaves, the sub-aSrial parts of which possess the properties of green foliage 

 leaves : our common kitchen Onion presents the most familiar example. As may 

 be readily supposed, subterranean shoots may resemble true roots not only in their 

 form but also in their functions; since they, like the latter, take up food from 

 the earth, and are endovs^ed with irritabilities by fteans of which they grow through 

 the substratum in all directions like true roots. We find subterranean shoots of this 

 kind sometimes covered with absorbing root-hairs, e. g. in the Horsetails. Besides 

 the horizontal creeping runners, there are others which penetrate obliquely or vertically 

 downwards into the earth, and behave also in other respects like primary roots. This 



occurs to a very marked extent in the subter- 

 ranean shoots of the genus Dracana; and from 

 the subterranean shoot-system of Equiselum 

 spring single branches, which grow down 

 2-3 m. perpendicularly into the earth. Since, 

 consequently, shoot-axes can undertake the 

 functions of roots, the development of true 

 roots may be entirely wanting in such cases. 

 There is a whole series of examples known 

 in this connection. 



The genus Psilotum, belonging to the 

 LycopodiacecB, developes a subterranean sys- 

 tem of shoots, the leaf rudiments of which 

 are only to be traced by careful microscopic 

 research; while they behave in other re- 

 spects exactly like roots. Completely rootless 

 are, further, some Orchidese devoid of chlo- 

 rophyll {Corallorhiza, Epipogum), the sub- 

 terranean shoots of which may in truth be 

 recognised as such at once, but nevertheless 

 completely undertake the fianction of roots. 

 I take this opportunity of remarking that 

 the genus Salvinia is also completely root- 

 less; in a manner, however, quite different 

 from the plants hitherto named. Salvinia is, 

 in fact, a plant floating horizontally on the 

 surface of water, and provided with broad aerial leaves, from the shoot axis of which 

 long root-like fibres hang down into the water: the latter are, however, neither true 

 roots nor metamorphosed shoots, but, according to the history of development, 

 peculiarly modified leaves, which have thus here undertaken the function of roots. 



In all the cases considered hitherto, we have been concerned with derived or 

 metamorphosed forms of shoot of those vascular plants in which at least the main 

 shoots either produce green assimilating leaves, or are abundantly provided with 

 chlorophyll in their stems. The departures from the type are in these cases brought 

 about by division of the physiological labour. The deviations from the typical 

 forms of shoot proceed much further in plants devoid of chlorophyll, which either 



Fig. 37-— a leaf of Allium cepa divided leng^thwise. 

 X the thickened baseof the sheath, which remains behind 

 as a bulb scale after the upper part of the leaf has died 

 down ; j the membranous part of the sheath ; / the hollow 

 lamina ; A cavity, and i inner side of the lamina ; x ligule. 



