J Syst Evol ›› 1981, Vol. 19 ›› Issue (2): 179-185.

• Research Articles •

### Evolutionary trends in secondary xylem of Gymnosperms

Yu Cheng-Hong

• Online:1981-05-18 Published:1981-05-18

Abstract: Basing on the evidence accumulated during the last few decades in the fields of plant anatomy and paleobotany, the present paper gives a summary in this regard as follows: 1. Scalariform tracheids are more primitive than pitted ones. 2. Torus on the membrane of a bordered pit-pair is a later evolved structure than none on it. 3. Fossil records show that the sequence of evolution in tracheary pitting is from the araucarian type to the coniferous one. 4. Tracheids without crassulae precede those with them. 5. Presence of spiral thickenings on tracheid walls does not show an advanced phenomenon, as in angiosperms, but a conservative one. 6. Absence of wood parenchyma is primitive; sporadically terminal diffuse represents the primitive type of its distribution; diffuse and diffuse in aggregates distributed throughout a growth ring are advanced and more advanced. 7. Ray tracheids have been observed even in Silurian specimens, it may be better to regard them as primitive rather advanced. 8. Resin canals have never been found in Paleozoic fossils but in Mesozoic ones.They are not so primitive as Jeffrey[26]suggested.

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