Chapter 5. Seeds and the Gymnosperms

 

Seedless vascular plants, featured in the previous chapter, were still dependent on free water for dispersal of the sperm cells and fertilization of the egg.  That hurdle was finally cleared by the evolution of pollen grains that carried sperm cells through the air, and ovules that enclosed the egg and could also be held high above the ground. After fertilization, the ovule becomes the seed, a complex organ containing the young sporophyte embryo that can also be dispersed through dry air. 

1. The first plants to produce seeds were gymnosperms ("naked seeds") in reference to the seeds not being enclosed in fruits. 

 2. The pollen grain consists of a tiny, sperm-producing gametophyte that stays within the wall of a microspore. Upon contact with an ovule, the pollen grain germinates by producing a pollen tube that carries the sperm cells to the vicinity of an egg.

3. The ovule consists of protective layers called integuments, surrounding a megasporangium with a single megaspore. The megaspore develops into a small egg-producing gametophyte that stays within the integuments as the ovule develops into the seed.

4. The early seed plants diversified into a wide variety of gymnosperms, some with compound, fern-like  fronds with microsporangia and megasporangia borne on leaves, some with simpler, fan-shaped or needle-like leaves, with microsporangia and megasporangia borne in catkins or cones (strobili) .

5, Angiosperms ("enclosed seeds") evolved from a fern-like gymnosperm as the leaf-like organs bearing  microsporangia and megasporangia became condensed into complex strobili called flowers.

 

 

 

Figure 5.1. With secondary growth in stems and roots, and freedom from water reproduction, 

seed plants were poised to become some of the most massive living organisms ever. 

These Coastal Redwoods  are  more than 100 meters tall. 

Figure 5.2. Among seedless vascular plants, Selaginella (A) provides an analog of how 

ovules and pollen might have evolved, but was not directly ancestral to seed plants. Here, 

both male (B) and female (C) gametophytes mature within their spore walls. After dispersal, 

sperm cells emerge from the small spore containing the highly reduced male gametophyte (B)

 and swim to a female gametophyte (C) with archegonia (egg chambers) exposed through

 the cracking spore wall. The female gametophyte contains considerable stored food, which 

supports the emerging sporophyte (D), much like a seed does. Drawings from Brown 1935.

Figure 5.3. Pollen grains are microspores, within which a highly reduced male gametophyte 

develops. The pollen grain of a pine tree (A) , has two ear-like air sacs to aid in wind-dispersal. 

The enclosed gametophyte consists of two functional cells, a tube cell (lower) and a 

generative cell (middle), plus the degenerated remains of two vegetative cells (upper). 

Upon germination, the tube cell develops into a pollen tube (B), and the generative cell 

divides to form two sperm cells that migrate through the pollen tube toward the egg.

 In cycads and ginkgoes, sperm cells are peculiarly multiflagellate, reflecting a more ancient 

fertilization process. in which sperm cells have to swim on their own to reach the egg.

 Drawings from Brown 1935 (A, B) and Coulter 1910 (C).

Figure 5.4. An ovule houses the female side of plant reproduction, with the prefix "mega" signifying female. It begins as a megasporangium protected within a protective integument. The megasporangium contains a single megaspore (the female spore), surrounded by the food-storage tissue, nucellus. The megaspore then develops into the  megagametophyte with two eggs at the upper end. One of the two eggs is fertilized and develops into a young sporophyte embryo, while the gametophyte tissue develops into food storage for the embryo. The mature structure at that point is the seed.  

Figure 5.5. The embryo of vascular plants consists of an axis, with a shoot apex at one end 

and a root apex at the other end. A suspensor is often present, as in the Selaginella embryo (A), 

serving  to push the embryo deeper into the food storage tissue. In many seed plants,  

the embryo is suspended in food storage tissue from the gametophyte tissue in gymnosperms (B),

 (or endosperm in angiosperms), while in some specialized seed plants, like the bean (C),

the stored nutrients are transferred entirely to the two seedling leaves (cotyledons) as the seed ripens. 

Drawings from Haupt 1953 (A, B) and Brown 1935 (C).

Figure 5.6 The first true seed plants were seed ferns (A, Lygenopteris oldhamia), with

 pollen sacs (microsporangia) and ovules (megasporangia) borne directly on large compound 

fronds (B, Sphenopteris tenuis). The ovules of Genomosperma (C) had an incompletely

 developed integument consisting of a ring of leaf segments. Drawings from Brown 1935.

Figure 5.7. Cordaites was an early conifer with simple strap-shaped leaves and 

reproductive structures borne in simple cones (strobili). Drawing from Haupt 1953.  

Figure 5.8. The leaves of the ancient ginkgophyte, Baiera gracilis (A), was divided, 

with forking subdivisions. The lone survivor of this lineage, Ginkgo biloba, has fan-shaped 

leaves with forking (dichotomous) veins. Pollen sacs are borne in slender catkins 

(flexible strobili) while naked ovules are borne on short stalks. 

Drawings from Brown 1935.

Figure 5.9. Conifers are diverse and abundant today, with leaves needle-like, scale-like, or 

strap-shaped. In pines (A) and false cypress (C), seeds are borne in woody cones, while in  

Taxus (Yew) (B) cones are absent, and the solitary seeds are surrounded by a red, fruit-like aril, 

apparently a relatively recent adaptation for bird-dispersal. 

Drawings from Brown 1935 (A< B) and Kerner and Oliver 1895 (C).

Figure 5.10. Agathis is a conifer from the southern hemisphere, with broad leaves and 

massive seed cones. Drawing from Brown 1935.

Figure 5.11 Gnetophytes are an odd assemblage of three genera with complex male and 

female strobili. Gnetum has broad, reticulately-veined leaves (A) similar to many angiosperms,

 but simple pollen-bearing structures (B) arranged in complex clusters around the nodes of a 

catkin (C). Ovules are similarly arranged (D) and ripen into red, berry like naked seeds (E). 

Drawings from Brown 1935. 

Figure 5.12. Ephedra, the second genus of the gnetophytes, consists of leafless green-stemmed 

desert plants with small cones at the nodes bearing either stalked pollen sacs (B) or ovules (C). 

Drawings from Brown 1935.

Figure 5.13. The third genus of the gnetophytes is the bizarre Welwitschia. The single species

 in the genus lives in the dry Namibian Desert in southwestern Africa. The plant has only 

two opposite leaves for its entire life, which grow slowly in length from basal intercalary 

meristems at their bases. Seed and pollen cones are produced on separate male and female 

plants from axillary meristems at the rim of the stem, which has no apical meristem or

 upward growth. Drawing from LeMaut and Decaisne 1876.

Figure 5.13A. Welwitschia in its natural habitat. In the older plants, the two opposite leaves 

have split into long parallel segments. The plants have both long taproots that can reach 

underground water sources and shallow roots that can absorb drops of dew that fall from 

moisture collected from the fog by the leaves. A.I. reconstruction.

Figure 5.14 Cycads are an ancient lineage of gymnosperms with a number of surviving species.

 The are superficially palm-like with large compound leaves and reproductive structures 

borne in loose strobili. Drawing from Brown 1935.