590 



BOTANY OF THE LIVING PLANT 



frequent feature in sporelings and embryos, and it also appears in 

 many adult plants. But the result is unpractical, both mechanically 

 in regard to stability, and physiologically in regard to the absorption 

 and transmission of materials. Plants so constructed present an ever 

 more insistent problem of well-being as their size increases. 



Conical and Obconical Form Contrasted. 



The prevalence of this type in plants of primary construction is 



apt to be lost sight of by the student, owing to the customary practice 



of presenting first to him such relatively advanced examples 



as Dicotyledons and Gymnosperms. His mind is thus familiarised 



with the idea of conical form tapering upwards, as seen in any forest 



tree : and especially in the Coniferae, such as Sequoia. It is also 



conveyed by descriptions of cambial thickening (Chap. IV.), or by 



A BCD 



r 



Fig. 446. 



Diagrams not drawn to uniform scale, showing various methods of development 

 of plants of primary obconical construction as their size increases. A = Plant of 

 Maize or of Screw Pine, with prop-roots. B =Stem of Cocos or Oreodoxa showing the 

 widely obconical base with attached roots, followed by the cylindrical trunk. 

 C = Dracaena, with obconical primary development, supported by secondary thicken- 

 ing. D =Tree Fern, with its obconical stem supported by a massive sheath of roots. 



comparison of sections of woody stems taken successively from 

 below: or again by diagrams like those of Fig. 37. But timber trees 

 are products of relatively advanced evolution, involving secondary, 

 that is, cambial activity. The student should therefore distinguish 

 clearly between such conical stems, and the obconical contours that 

 are apt to follow on primary development. Examples of these, which 

 have retained that primary plan unaltered to the adult state, are 

 shown diagrammatically for well-known plants in Fig. 446. The 

 rubrics suggest how each has made the best of this seemingly dis- 

 advantageous scheme as its size increases. There are, in point of fact, 

 two outstanding types of organisation of dendroid plants : the one 

 having an obconical primary stem, which bears an enlarging distal 



