HAIRS (TRICHOMES). 



147 



of the tissue of the root, which it is desirable to 

 of the formation of pith which is usually entirely 



simpler cases of the differentiation 

 mention here especially on account 

 absent from slender roots. 

 Generally also the pith dis- 

 appears in thicker roots 

 when they become more 

 slender as they increase in 

 length; the hollow cylinder 

 of fibro - vascular tissue 

 ends in a solid bundle. 



(e) The roots are ge- 

 nerally clearly distinguished 

 from the leaf- forming 

 shoots by the characteristics 

 mentioned above ; there 

 occur, however, a few tran- 

 sit ion al forms •^vhich show 

 that roots can become di- 

 rectly transformed into 

 leaf- bearing shoots, as in 

 Neottia nidus-a'vis, where 

 older lateral roots of the 

 stem throw off their root- 

 caps and form leaves be- 

 neath the apex (Reichen- 

 bach, Irmisch, Prillieux, 

 Hofmeister). On the other 

 hand, leaf -forming shoots 

 cease to produce leaves, 

 as in many Hymenophyl- 

 laceae (according to INIct- 

 tenius) ; these leafless 



growing shoots form root-hairs, and assume the habit of true roots (whether they 

 actually form a root-cap is doubtful); in these species true roots are wanting. In 

 Psilotum triquetrum Naigeli and Leitgeb have shown that the apparent roots are only 

 underground shoots, on which more or less evident traces of leaf-formation may be 

 recognised ; they are similar to true roots in function and in the formation of tissue, 

 but have no root-cap, and, when they come to the light above ground, continue to grow 

 in the manner of ordinary leaf-shoots. In the Selaginella: also, the same investigators 

 have shown the presence of leafless shoots (root-bearers) which grow downwards, and 

 do not form root-caps until they touch the ground (cf. Book II. Lycopodiaceae). 



We thus see that transitional formations between roots and leaf-shoots are found 

 even in highly differentiated plants. But even in Algae the thallus is often fixed to 

 its substratum by organs of attachment which may be compared with roots in their 

 habit and in many functional properties, not only in the case of the large Fucaceae and 

 Laminariae, but also in the unicellular Vaucheria and Caulerpa. 



In reference to the confirmation of the Theory of Descent brought forward at the 

 conclusion of this work, it is of great importance to know that members differing to the 

 greatest extent morphologically and physiologically are connected by transitional forms, 

 and that, especially in the branched thallomes of Algae, starting points are to be found for 

 all the differentiations of the higher plants. Distinctions which, in the ramifications of 

 the Alga-thallus, are only of a weak, undefined, and rudimentary character, increase more 

 and more in the higher plants ; points which can be sharply defined in the latter become 

 indistinguishable when we are considering the more simple Thallophytes. The more 



L 2 



FIG I 

 from the 

 ^ s bundl 



17. — Part of a transverse section of the primary root oi Zca Mais not far 

 apex ; e epidermis with its strongly swollen outer walls vd; p the cortex ; 

 ; sheath ; m the pith ; a- the thickening ring in whicli lie the vessels i^^. 



