210 VEGETABLE ORGANOGRAPHY. 



tlie roots of Hyacinths, which are growjn in transparent 

 glasses, — those which shoot out from Bhizophora, &c. — 

 those of plants which live in water, as Ranuculus aqua- 

 tilis, preserve a white silvery appearance, with the 

 exception of their extremity, which is sometimes green- 

 ish ; whilst on the other hand the stems and leaves ai'e 

 almost entirely green. From the roots never being 

 green, physiologists conclude that they do not decom- 

 pose carbonic acid gas, and do not disengage any oxygen 

 by the action of light. It is sufficient for me here to 

 remark this phenomenon, as a proof of the difference of 

 nature between roots and stems. 



The anatd^^isftl structure of roots in general is dis- 

 tinguished from thatjtstems by two prominent charac- 

 ters: — 1st, by the to^pi aosence of tracheae ; for all that 

 has been heretofore said of them in roots is found to be 

 incorrect; — 2d, by the total asbence of stomata. The 

 internal structure of roots, compared with that of stems, 

 does not present any other perceptible difference in 

 Endogens ; we remark here, in the same manner, fibres 

 composed of dotted or striped vessels, intermixed and 

 surrounded with cellular tissue. 



The similarity of the roots and stems is not found in 

 Exogens ; the medullary canal, which in these plants 

 accompanies the stem through its whole length, stops 

 suddenly at the neck, where it forms a cul-de-sac, the 

 root being entirely devoid of pith ; Grew and Malpighi 

 observed it in some plants, such as the Borage, Chicor}'^, 

 Tobacco, &c. But although the roots of Exogens are 

 devoid of pith, the medullary rays are found diverging 

 from the centre to the circumference, and often more 

 distinct than in the stem, as may be seen in the Radish 

 and Carrot. The woody body of the roots of Exogens 

 is smaller in proportion than in the stem; but the 

 absence of the pith seems compensated by the great 



