168 XII. STRUCTURE OF ROOTS. 



throughout, without disadvantage. If the thickening takes place 

 early, then water transit may be effected by means of the passage- 

 cells, which, as we saw above in Iris, are placed opposite to the 

 groups of water-conducting vessels in the central cylinder. 



Root of Ranunculus repens. The roots of Dicotyledons are 

 less favourable for study than those of Monocotyledons ; but 

 now that we have obtained an insight into these latter, it will not 

 be difficult correctly to interpret the former. We prepare a cross- 

 section from the base of a strong adventitious root of the runner 

 of the Creeping Buttercup (Ranunculus repens) abundant every- 

 where by roadsides and in pastures, and as a garden weed in 

 damp soils. The central cylinder, or stele, does not appear so 

 sharply denned towards the cortical tissue as it is in Monocotyle- 

 dons, but with careful observation we shall find, at the junction 

 of the two, the endodermis, marked with its dark shading; for 

 a thickening endodermis, so common in Monocotyledons, is more 

 exceptional in Dicotyledons, and this absence stands manifestly 

 in relation with the power of secondary growth which their 

 central cylinder in general, though not in this particular case, 

 possesses. According to the strength of the root, the wood in 

 the central cylinder is in four or five rays ; the great vessels here 

 also lie towards ,-the centre, the small ones outside them. In 

 Monocotyledons an innermost vessel is often distinguished by 

 its exceptional size ; in Dicotyledons this is seldom the case, and 

 is not to be observed in Ranunculus. 1 The xylem rays in Ranun- 

 culus extend to the centre of the stele, and there coalesce more or 

 less completely with one another. Yet, if at all, the innermost 

 vessels are only fully formed quite late, and remain mostly in the 

 state of thin-walled, elongated cells. The bast bundles alternate 

 with those of the wood in the ordinary way. . 



Roots of Vascular Cryptogams. The roots of Vascular Crypto- 

 gams are constructed upon the same fundamental type as those of 

 Phanerogams. 



Secondary Thickening in Tawus baccata. We will follow out 

 in Taxus baccata (the Yew) the processes which lead to the 

 secondary increase in thickness of those roots of Dicotyledons 

 and Gymnosperms which are capable of it. For this purpose we 

 procure a piece of root with young uninjured branchlets. We take 



1 In the adventitious roots upon the rhizome of R. repens, according to De 

 Bary, Comparative Anat. (Eng. trans., pp. 355-6, and Fig. 165), the axis of the 

 cylinder is occupied by one large vessel or pitted duct. [Eo.] 



