VIII] SUPPORT BY ADVENTITIOUS ROOTS 159 



problems are solved in both cases by adventitious roots. In most Palms the 

 cone widens rapidly at the base, and the strut-roots arise near the base only. 

 Sometimes, as in Verschajfeltia, they spring far up the elongated conical stem. 

 This latter state is seen in Tree Ferns. Their stem gradually increases from 

 its narrow base upwards, till the full size of the adult tuft of leaves is reached. 

 It then continues approximately as a cylinder. The lower conical region is, 

 however, buttressed by the development of innumerable adventitious roots, 

 which, growing down, interlacing, and developing with sclerotic cortex, con- 

 stitute not only an efficient mechanical support, thus making up for the 

 weakness of the slender obconical stem, but also yield the increased physio- 

 logical supply needed by the enlarged area of leaves (Fig. 152). With such aid 

 from the adventitious roots a primary development of the shoot, which would 

 otherwise have been an unworkable proposition, becomes mechanically and 

 physiologically sufficient for upright stems, without resort to secondary 

 increase. Most Ferns, however, have a creeping habit, so that in them the 

 mechanical difficulty does not arise. 



From the phyletic point of view the argument to be based on the stelar 

 structure seen in Leptosporangiate Ferns is the same as that for the more 

 primitive Ferns described in Chapter VII. Those which in the adult state are 

 structurally nearest to the protostele are held to be relatively primitive in respect 

 of that feature; those which have departed furthest from it are regarded as 

 the most advanced. But since similar advances are found in Ferns divergent 

 in other respects, it appears that they are liable to homoplastic development. 

 Hence similarities in advanced structure cannot themselves alone be held as 

 trustworthy indications of affinity. 



Note. To avoid needless repetition the bibliography of stelar tissues will be given collectively 

 for Chapters vii, viii, and x at the close of Chapter x. 



