The seed plants (Subphylum Spermatophytina, Phylum Tracheophyta) comprise seed-bearing vascular plants, of which more than 300,000 species of extant seed-bearing vascular plants are known (Encyclopaedia Britannica). They represent a clade which is known as the Spermatophyta and consists of the gymnosperms and the angiosperms.

Summaries of the phylogenies of the gymnosperm and angiosperm clades are shown below:
​Figure 1. Summarized phylogenetic tree of the gymnosperms
​Figure 2. Summarized phylogenetic tree of the angiosperms
​For ease of reference, the stem lines shown in the above trees are numbered as shown below:
Figure 3. Branch numbering scheme for the gymnosperms
Figure 4. Branch numbering scheme for the angiosperms
​The pages corresponding to the branches shown can be found by clicking the links in the following table:
Branch number
Branch name
Page link
2-28
Stem-Spermatophyta
2-28-1
Stem-Gymnosperms
2-28-2
Stem-(Ginkgoales+Cycadales)
2-28-3
Stem-Ginkgoales
2-28-4
Stem-Cycadales
2-28-5
Stem-Pinopsida
2-28-6
Stem-(Cupressales+Araucariales)
2-28-7
Stem-Cupressales
Value
2-28-8
Stem-Araucariales
2-28-9
Stem-(Pinales+Gnetidae)
2-28-10
Stem-Pinales
2-28-11
Stem-Gnetidae
2-28-12
Stem-Ephedrales
2-28-13
Stem-(Welwitschiales+Gnetales)
2-28-14
Stem-Welwitschiales
2-28-15
Stem-Gnetales
2-28-16
Stem-Angiosperms
2-28-17
Stem-Amborellales
2-28-18
Stem-(Angiosperms less Amborellales)
2-28-19
Stem-Nymphaeales
2-28-20
Stem-(Angiosperms less Amborellales & Nymphaeales)
2-28-21
Stem-Austrobaileyales
2-28-22
​Stem-(Angiosperms less Amborellales & Nymphaeales & Austrobaileyales)
​2-28-23
Stem-Chloranthales
2-28-24
​​Stem-(Angiosperms less Amborellales & Nymphaeales & Austrobaileyales & Chloranthales)
​2-28-25
Stem-Magnolianae
2-28-26
Stem-(Magnoliales+Laurales)
2-28-27
Stem-Magnoliales
2-28-28
Stem-Laurales
2-28-29
Stem-(Piperales+Canellales)
2-28-30
Stem-Piperales
2-28-31
Stem-Canellales
2-28-32
Stem-(Lilianae+Ceratophyllales+
​eucotyledons)
2-28-33
Stem-Lilianae
2-28-34
Stem-(Ceratophyllales+eucotyledons)
2-28-35
Stem-Ceratophyllales
2-28-36
Stem-eucotyledons
2-28-37
Stem-Ranunculales
2-28-38
​Stem-(eucotyledons less Ranunculales)
2-28-39
Stem-Proteanae
2-28-40
Stem-Sabiales
2-28-41
Stem-Proteales
2-28-42
Stem-(Trochodendrales+Buxales+core eudicots)
2-28-43
Stem-Trochodendrales
2-28-44
Stem-(Buxales+core eudicots)
2-28-45
Stem-Buxales
2-28-46
Stem-(core eudicots)
2-28-47
Stem-Gunnerales
2-28-48
Stem-Pentapetalae
2-28-49
Stem-(Dilleniales+superrosids)
2-28-50
Stem-Dilleniales
2-28-51
Stem-superrosids
2-28-52
Stem-Saxifragales
2-28-53
Stem-rosids
2-28-54
Stem-Vitales
2-28-55
Stem-Rosanae
2-28-56
Stem-superasterids
2-28-57
Stem-Santalales
2-28-58
Stem-(superasterids less   Santalales)
2-28-59
Stem-Berberidopsidales
2-28-60
Stem-(Caryophyllales+Asteranae)
2-28-61
Stem-Caryophyllales
2-28-62
Stem-Asteranae
This page thus covers Branch 2-28, along which are found the stem-group seed plants.

No recent publications seem to define the synapomorphies of the seed plants. However, the following characteristics of the extant spermatophytes are listed as synapomorphies in Bremer (1985):
  1. Vascular cambium (layer of actively dividing cells between xylem (wood) and phloem (bast) tissues that is responsible for the secondary growth of stems)
  2. Eustele (central part of the stem comprises a central ring of bundles around a pith)
  3. Embryogenesis with free-nuclear phase (repeated nuclear divisions occur without cell-wall formation)
  4. Single functional megaspore mother cell (produces four megaspores through meiosis)
  5. Integument (outer covering of the ovule that develops into the seed coat)
  6. Micropyle (a small opening in the surface of an ovule, through which the pollen tube penetrates)
  7. Linear tetrad of megaspores
  8. Seeds
These synapomorphies represent evolutionary novelties that occurred at some point during the stem-to-crown transition after the split from the ferns (Monilophyta).

An interpretation of the phylogeny of the stem-Spermatophyta, based on two publications, is shown in the time tree below:
​Figure 5. Time tree of the stem-Spermatophyta
​The oldest known member of the seed plant stem group is Aneurophyton germanicum, described from Middle Devonian (Late Eifelian) sediments in the Massif de la Vesdre at Goé, Belgium (Momont et al, 2012; Toledo et al, 2021). This species is illustrated below, together with other stem group fossils for which images are available in the public domain (click on image for a larger view):
​​Figure 6. Images of stem-Spermatophyta
​The above images are ordered from most basal to most crownward, and it seems possible to suggest tentatively that there is a trend from primitive-looking trees such as Aneurophyton through Tetraxylopteris to trees that look somewhat more like those of the present day (Archaeopteris).

As indicated in Figure 1, the spermatophyte stem-to-crown transition took place over a period of at least 47 million years, from Middle Devonian to Late Carboniferous time. However, the highest rate of evolutionary change took place during the Middle Devonian, as indicated below:
Figure 7.   Rate of appearance of stem-group spermatophytes (predating the crown group and including only the species shown in Figure 1)

References

Bremer, K. (1985). Summary of green plant phylogeny and classification. Cladistics, 1(4), 369-385.

Momont, N., Gerrienne, P., & Prestianni, C. (2012). Aneurophyton germanicum (Aneurophytales-Progymnosperms) from the Middle Devonian of Belgium and Germany. Japanese Journal of Palynology, 58 (Special), 156-157.

Toledo, S., Bippus, A. C., Atkinson, B. A., Bronson, A. W., & Tomescu, A. M. (2021). Taxon sampling and alternative hypotheses of relationships in the euphyllophyte plexus that gave rise to seed plants: insights from an Early Devonian radiatopsid. New Phytologist, 232(2), 914-927.
Image credits – stem-Spermatophyta
  • Figure 6   (Rellimia sp.): From Open Access article Harrison, C. J., & Morris, J. L. (2018). The origin and early evolution of vascular plant shoots and leaves. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20160496.
  • Figure 6    (Aneurophyton germanicum, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 6    (Aneurophyton germanicum, life restoration): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons 
  • Figure 6    (Tetraxylopteris sp., fossil): Skye McDavid, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons 
  • Figure 6    (Tetraxylopteris sp., life restoration): Falconaumanni, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 6    (Archaeopteris macilenta, fossil): Photographed by Bob James (owner of website) at Denver Museum of Nature & Science, August 2023.
  • Figure 6    (Archaeopteris macilenta, life restoration): Retallack, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 6    (Elkinsia sp.): Zeiram1990 aka Georgi Ivanov on Fandom under CC BY-SA license
  • Figure 6    (Heterangium americanum): Photo by John Hall, Botanical Society of America, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
  • Figure 6    (Lyginopteris baeumleri): Daderot, CC0, via Wikimedia Commons
  • Figure 6    (Medullosa stellata): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons