Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain.

  • Coulter, John Merle, 1851-1928.
Date:
[1910]
Catalogue details

Licence: In copyright

Credit: Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain. Source: Wellcome Collection.

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    pteridophytes, as for example in Osmundaceae among ferns. In the usual sequence of fern families, the polystelic condition is found at both extremes; and the confessedly primitive protostelic condition persists in the midst of the fern series. The inference is that in various ways, along various lines, changes in the primitive protostelic con- dition have, developed. To arrange all the types of stele in a linear, phylogenetic series seems to be not only unnecessary, but also con- trary to all previous experience with phylogenies. Assuming that the polystele is merely a split-up siphonostele (31, 36), there are just two types of central cylinder, the protostele and the siphonostele, the former being the more primitive. Jeffrey has shown (31) that siphonosteles are of two fundamental types: (1) those with both leaf and branch gaps (phyllosiphonic), and (2) those with branch gaps alone (clad 0 si phonic). The phyllosiphonic condition is associated with large leaves, and the cladosiphonic with small ones; and therefore Jeffrey recognizes two great primitive stocks of vascular plants: the Lycopsida, which are cladosiphonic and microphyllous; and the Pteropsida, which are phyllosiphonic and megaphyllous. To the latter stock belong the ferns, gymno- sperms, and angiosperms. This association of ferns and gymno- sperms is a strong confirmation of the conclusion reached in other ways as to their phylogenetic connection. The leaf gap, characteristic of the Pteropsida, should be under- stood clearly. A transverse section of the vascular cylinder of Adian- tum, passing through a leaf gap, shows two crescentic vascular strands facing each other (fig. 6), one being a section of the leaf trace, the other a section of the siphonostele which the leaf gap has made incom- plete. At the edge of the gap the external and internal phloems become continuous. Farther up the stem the gap closes, and the cylinder becomes complete again. There is sometimes danger of mistaking a branch gap for a leaf gap, but the latter should be recog- nized by the fact that it occurs immediately above the leaf trace to which it belongs. A true leaf gap is always related to a single trace, so that if several traces appear in relation to a gap in the vascular cylinder, and especially if they are related to the sides of the gap, it is not a true leaf gap. It should also be noted that gaps in the vascu- lar cylinder may occur where the vascular tissues are much reduced