599 



which, in situations where it is in contact with the bundle, forms a 

 hard brown or blackish layer. In some ferns and Lycopodiaceae the 

 fibro-vascular bundles lie within the external cellular portion of the 

 stem for some distance before they become free in the roots. In old 

 stems of some Lycopodiaceae this parenchyma is separated from the 

 central fibro-vascular column, and such descending bundles are found 

 lying in the interval. 



" Monocoiyledonous Roots. — The radicle of germinating monocoty- 

 ledons does not appear as a direct continuation of the stem, but 

 breaks through the radicular extremity of the embryo. A layer of the 

 superficial cellular tissue clothes this radicle as with a sheath at its 

 base. Hence the embryos of this class have been called endorhizae, 

 and the sheath formed by the radicular extremity is termed the coleo- 

 rhiza. 



" The roots of monocotyledons are usually compound, and the 

 branches are occasionally again divided. These partial roots, form- 

 ing together the compound, are only temporary formations ; if the 

 stem is perennial they die in the order of their production, from the 

 centre outward, new roots being developed in circles continually more 

 external. The aerial roots commonly occurring in this class fre- 

 quently exhibit this phenomenon. They are produced from various 

 parts of the stem : in the palms the base of the stem becomes cover- 

 ed by the production of a large number of these adventitious or free 

 roots, giving the base a conical form ; in the Pandanus they are de- 

 veloped in a continually enlarging spiral, the most recently formed 

 being the largest, and rising high up on the stem ; the death of the 

 lower roots and the gradual destruction of the base of the stem often 

 cause these free roots to become finally almost the sole connexion be- 

 tween the stem and the earth. In Dracaena the descending portion 

 of the fibro-vascular bundles which belong to the radicle system do 

 not become fi-ee and develope adventitious roots upon the stem, but 

 are found as wood beneath the cortical layer, until they reach the 

 base of the stem, where they are united to a certain extent by paren- 

 chyma, so as to present a root somewhat resembling that of a dicoty- 

 ledonous tap-root. Cordyline australis has two such main divisions, 

 called by Mirbel souches. Here, however, the monocotyledonous 

 structure is retained, the radicles are developed centrifugally, and 

 break through the cortical parenchyma, which clothes them with a 

 coleorhiza. The flattened stems in the bulbs of our climates exhibit 

 also the centrifugal development of the radicles. 



" The anatomical structure corresponds to that of the stems. The 



