CH. xm] MEANING OF LIGNIFI CATION 173 



ence being that the xylem tends to be reduced in amount, while 

 the lignification is often very poor. Spiral or annular vessels, 

 when present in the neighbourhood of the growing apex, may, 

 in some instances, be completely destroyed by the elongation of 

 the internodes, and may survive only at the nodes, e.g. Potamogeton 

 /ucens 1 , Zannichellia palustris 1 ^ Althenia filiformis 2 , etc., while 

 in the case of Elodea canadensis^ the tracheal thickenings do not 

 even persist in the nodal tissues. Ceratophyllum (Fig. 56, p. 87) 

 is an example of a further degree of reduction, since here ligni- 

 fication is entirely lacking, even in the apical region. This loss 

 of lignification has been sometimes regarded as a corroboration 

 of the widely-held view that the transpiration stream has no 

 existence in submerged plants. But, as we shall show in Chapter 

 xxi, the tftea that such a current is absent in these plants, seems 

 often to have been accepted on totally inadequate grounds. In 



FIG. 112. Hippuris vulgaris, L. Diagram of the horizontal rhizome as seen from 

 above to show sympodial growth ; a-A ; b-B ; c-C ; d-D, E, represent successive 

 axes. [Irmisch, T. (1854).] 



this connexion it appears to the present writer that, when xylem 

 and the part which it plays in water-conduction is being con- 

 sidered, too much stress is often laid almost unconsciously 

 perhaps on the question of lignification. It seems sometimes 

 to be assumed that the functional importance of the xylem is 

 proportional to its degree of lignification; an idea which may 

 perhaps be interpreted partly as a hypnotic impression con- 

 veyed to the botanist's mind by the vividness of the xylem in 



Caspary, R. (18582). 



Prillieux, E. (1864). 



