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BESSEY PHYLOGENETIC TAXONOMY 113 



2. In general, homogeneous structures (with many and 



similar parts) are lower, and heterogeneous structures 

 (with fewer and dissimilar parts) are higher. 



3. Evolution does not necessarily involve all organs of the 



plant equally in any particular period, and one organ 

 may be advancing while another is retrograding. 



4. Upward development is sometimes through an increase 



in complexity, and sometimes by a simplification of an 

 organ or a set of organs. 



5. Evolution has generally been consistent, and when a par- 



ticular progression or retrogression has set in, it is per- 

 sisted in to the end of the phylum. 



6. In any phylum the holophytic (chlorophyll-green) plants 



precede the colorless (hysterophytic) plants, and the 

 latter are derived from the former. 



7. Plant relationships are up and down the genetic lines, and 



these must constitute the framework of phylogenetic 

 taxonomy. 



B. DICTA HAVING SPECIAL REFERENCE TO THE GENERAL 



STRUCTURE OF THE FLOWERING PLANTS 



8. The stem structure with collateral vascular bundles ar- 



ranged in a cylinder is more primitive than that with 

 scattered bundles, and the latter are to be regarded as 

 derived from the former. 



9. Woody stems (as of trees) are more primitive than her- 



baceous stems, and herbs are held to have been derived 

 from trees. 



10. The simple, unbranched stem is an earlier type, from which 



branching stems have been derived. 



11. Historically the arrangement of leaves in pairs on the 



stem is held to have preceded the spiral arrangement 

 in which the leaves are solitary at the nodes. 



12. Historically simple leaves preceded branched (* 



com 



pound 



13. Historically leaves were first persistent ("evergreen") 



and later deciduous. 



14. The reticulated venation of leaves is the normal structure, 



