596 



BOTANY OF THE LIVING PLANT 



point of a twig lies buried as a minute speck within the bud itself, 

 which is less than three millimetres in diameter. But the Beech has 



the advantage of cambial thickening, which 

 Ferns and Palms have not. Hence the pro- 

 blem of provision of channels of supply to 

 stem and leaves differs in the two types : the 

 one shows " exogenous " growth, after the 

 old terminology : the other is " endogenous ". 

 While the former depends upon secondary 

 cambial increase and accretion of successive 

 layers of conducting tissue in the woody 

 trunk, the latter shows distal expansion of the 

 primary stele. The first of these modes of 

 increase has been described at length on pp. 

 55 to 68 : the latter is less clearly under- 

 stood, and it will be reconsidered here, as 

 it is seen in Ferns, and in Palms. 



The general structure of an adult Palm stem, 



as seen in transverse and longitudinal sections, 



Winter-buds of the Beech (Fagus with its distended and ill-defined stele, and 



S^jfSSl^g^SS numerous vascular strands scattered through the 



proportion of exposed surface in pith, leads out to the successive leaf-bases, as 



the defoliated state. Natural size. , ., , . ™, ., , « -i— 



(After Strasburger.) described m most Textbooks (see p. 50, rigs. 29, 



30). But few botanists who are familiar with the 



structure of an adult Palm or Maize plant could give a detailed account of 



the development of their vascular systems from the seedling upwards : few 



also will have themselves dissected the adult buds, or have realised that 



Fig. 452. 



Loxsoma Cunninghami. Diagram showing the form of the medullated stele at 

 a node of the rhizome, ss = solenostele ; U= departing leaf- trace ; lg = leaf -gap. 

 The arrow points towards the apex of the rhizome. (After Gwynne-Vaughan.) 



greater or less expansion of the primary stele in Monocotyledons stands in 

 a direct relationship to formative activities in the enlarging distal bud, and 

 its developing leaves. The stele, moreover, is responsible for distributing 



