The gametophytes of vascular plants are even simpler than those of bryophytes. Yet we’ve already seen that vascular plants are larger and more complex than nonvascular plants. In the vascular plants it is the sporophyte generation that dominates, becoming large, independent, long lived, and able to produce many sporangia over its lifetime. All trees, shrubs, and larger herbs are vascular plant sporophytes.
This fourth lineage of land plants featuring dominant sporophytes is tech- nically referred to as the Polysporangiophyta, for the primary adaptation of their common ancestor was to expand spore production through branching and formation of many sporangia. The earliest members of this group, such as Aglaophyton (Fig. 3.13), were small and did not yet have vascular tissues. Xylem and phloem evolved somewhat later, and this is what allowed diploid sporo- phytes to evolve into larger, more aggressive forms. Plants like Aglaophyton, however, successfully exploited the ability for increased spore production and set the stage for the vascular plants.
Ferns are abundant and diverse today, but they represent well the form and life cycle of vascular plants before the advent of seeds. The fern sporophyte is the long-lived generation, living many years and producing a great many spores. The gametophyte, however, is fundamentally similar in structure and function to those of some nonvascular plants, resembling a short-lived liverwort (See Fig. 3.4). Gametophytes are even smaller in seed plants, though still present, hidden within pollen grains and young seeds.
There is still a mystery as to exactly how the vascular plants got started, or more precisely, how their robust, independent sporophytes got started. It was not simply a matter of a moss or liverwort sporophyte detaching itself from its gametophyte and getting taller. The pattern of growth is fundamentally different. The sporophytes of nonvascular plants are simple, unbranched, and produce a single sporangium, while those of the vascular plants are able to both grow and branch by means of apical meristems—dividing cells at the tips of stems.
It has been suggested that the sporophyte of vascular plants came about as the ability to continue growth and branching evolved in the sporangium stalk of some ancient bryophyte. This is known as the antithetic or interpolation hypoth- esis of sporophyte evolution. Ligrone et al. (2012) outlined a more sophisticated version of this scenario.
How would the simple stalk of a bryophyte start branching? The stalk that lifts the sporangium of a moss or liverwort is essentially an “afterthought.” As we saw earlier, the sporangium of a moss begins its development through division of the zygote into a diploid mass. Later, a few cells between it and the parent plant begin dividing and expanding to produce the stalk. In the stalks of some modern moss sporophytes, cells have the extraordinary ability to expand up to 20 times their orig- inal length, but once that’s done, there’s no way for any new growth or branching to occur. So it would seem that nonvascular sporophytes early evolved into a dead end from which they were developmentally incapable of becoming larger, more com- plex, or independent.
Another hypothesis has been around for years, however. It proposes that the common ancestor of all land plants had an equal alternation of genera- tions, something like in the sea lettuce, Ulva. This is generally referred to the
Figure 3.13 A reconstruction of the prevascular plant, Aglaophyton major, which consisted of
homologous hypothesis of sporophyte origin. From such a beginning, sporo- phytes in nonvascular plants became simplified and those in the vascular plants became more complex.
By all accounts, however, the land plants are not related to Ulva or any other known alga that has an equal alternation of generations. The charophyte cous- ins of land plants, on the contrary, have no sporophyte generation at all. Their zygotes divide through meiosis to form haploid zoospores or new haploid plants. So the possible existence of equal generations in early land plants is something of a puzzle.
It is possible, however, that both of these hypotheses are partially true. The common ancestor of nonvascular and vascular plants was most likely a haploid plant similar to Coleochaete and simple liverworts. It grew and branched indefi- nitely from apical cells. It formed a simple diploid sporangium through division of the fertilized egg. In its bryophyte descendants, stalks formed but the sporophyte remained simple and dependent.
The branching vascular plant sporophyte may have evolved from this same starting point as the result of a genetic accident. In a bryophyte, the genetic machinery for branching and growth from apical meristems is well developed in the dominant gametophyte generation. The genes for this process are also present in the zygote, but they are normally turned off.
Suppose though, that in some ancient group of land plants, a mutation in a regulatory gene allowed the genes for branching growth to be turned on in the diploid embryo. It would then start growing and branching and would look pretty much like the haploid gametophyte, but would produce sporangia, as it was sup- posed to, instead of gametes.
At some point then, there may very well have been plants with very simi- lar gametophytes and sporophytes, but through a simple but dramatic genetic change, rather than through descent from algae that had an alternation of equal generations.
Support for this third scenario is coming from genetic research. We know that whole groups of organ-forming genes can be turned on or off by a single regula- tory gene. Many examples can be seen in the wealth of knowledge we now have of genetic development in plant models such as Arabidopsis. It has also been dem- onstrated that particular genes perform similar functions in both nonvascular gametophytes and vascular sporophytes. The development of root hairs (slender outgrowths of the root that enhance water absorption) in vascular plant sporo- phytes, for example, are controlled by the same genes that control the development of rhizoids (threadlike growths that provide anchorage) in moss gametophytes (Menand et al. 2007, Jones & Dolan 2012)). The next few years of genomic studies in both nonvascular and vascular plants may reveal more about the source of the genetic machinery that allowed vascular plant sporophytes to become indepen- dent, branching, long-lived plants.