THE PLANT BODY 



57 



sclerenchyma and sieve tubes. Sclerenchyma, where present in phloem 

 of simpler structure, consists chiefly of sclereids; fibers are few or 

 absent. The phloem of Drimijs is entirely "soft"; that of Zijgogi/num, in 

 the same family, has a few isolated fibers. Amborella has sclereids but 



Fig. 26. Transverse section of part of stem of Austrobailt'i/a scandens showing 

 secondary phloem with no companion cells. {From Bailey and Sivamy.) 



no fibers. Calycanfhus, on the other hand, has stratified phloem and 

 sieve tubes of high type. In herbs and subshrubs where there is little 

 phloem, the phloem is simple, usually without sclerenchyma — except as 

 a sheath or cap — and consists sometimes of sieve tubes and companion 

 cells only. The story of evolutionary modification seems to be one of 

 increase in complexity from a simple type, consisting wholly or largely 



