120 



ORGANOGRAPHY. 



neath it. Roots do not grow thei*efore, throughout their entire 

 length like stems, but only at their extremities, whieh are con- 

 tinually pushed forward and renewed. Every division of a 

 root also elongates in a similar manner. Eoots increase how- 

 ever, in diameter, in the same manner as stems. 



The points of tlie root Avere formerly considered to be special 

 organs, and called spo7igioles or spongelets {fig. 231, sp), from 



Fig. 231. 



Fig. 232. 



Fig. 231. Highly mognified vertical section of an Orchis root. gp. Spongi- 



ole. c,c. Parenchymatous cells, fj). Wood-cells and vessels Fig. 232. 



Fibrils or root-hairs on the surface of a young root. 



the idea that they absorbed fluid for the use of the plant, in 

 the same manner as a sponge sucks up water. But it will be 

 seen from the above description of the growth of roots, that 

 such ideas are entirely erroneous, for there are no special 

 org.ans or spongioles situated at the extremities of tlie root, but 

 that these arc only the denser layers of tissue which are gradu- 

 ally ])us]ied onwards by the development which is taking place 

 Avithin them, and as they decay tliey are rejjlaced by others. 

 It is also quite evident that absorption cannot take place to any 

 great extent, at least, through tliis old and dense tissue. 



At first tlic elongating growing extremities of the root consist 

 entirely of parencliymatous cells ; wood-cells and ves.'^els however 

 soon make their a]i])earance, and are constantly added to below 

 by the new tissue formed as the root continues to lengthen 

 (fg. 231, fp). When the root is fully developed, these vessels 

 and wood-cells geiu'rally form a central mass or wood, in which 

 there is commonly ]io pith, and no medullary sheath. liut 

 externally there is a true bark, which is also covered when 



