In some phylogenies, heterospory and the presence of a ligule are used to group certain taxa; in others herbaceous versus arborescent habit has been used to define hierarchy. Additional features found in ginkgo but not in phyla P1-1 through Pl-7 include anemophilous windborne pollen, sperm-conveying pollen tubes, haustorial pollination, ovules, and seeds. Tissues external to the cambium, collectively called bark, surround the trunk and branches. This represents an increase in efficiency over Pteridophytic reproduction. Seed-plant success reflects economic efficiency and the subsequent evolution of effective pollination syndromes, rather than integumentation of the ovule. Sphinxia possesses a single functional megaspore with associated aborted members of the meiotic tetrad, a distinct, striated sporangial wall, and a sterile envelope bearing proximal spines. This evolutionary success is due to the seed.
Answer Heterospory is a phenomenon in which two kinds of spores are borne by the same plant. Cone specimens with spores in situ indicate that, although all produced spores of the Calamospora type, some sporangia also contained spores belonging to the dispersed spore genera Cyclogranisporites, Vestispora, and Auroraspora. As the ginkgo ages, older xylem dies and becomes heartwood, a fibrous support tissue. The main evolutionary trends of the integument and ovule as a whole are considered. Another very important difference to gymnosperms is the angiosperm double fertilization.
Describe the advantages of seed habitat 7. Complete: Journals that are no longer published or that have been combined with another title. In this paper, we consider the suite of characters that define the seed habit, and discuss the probable selective pressures that produced each character. It begins with a discussion of geologic time, how organisms are preserved in the rock record, and how organisms are studied and interpreted and takes the student through all the relevant uses and interpretations of fossil plant. Genome sequences from ferns will provide the outgroup necessary to investigate the origins of these innovative features in a complete comparative framework.
. In these early ovules the nucellus was surrounded by integumentary tissue consisting. It has been suggested, however, that the megaspore in these lycopsids, unlike the megaspore of a seed, was shed from the sporangium at maturity in the typical free-sporing manner. Using sedimentological descriptions of the fluvial deposits and calculations of river discharge, palaeoclimatic trends as inferred from associated palaeosols will be further refined. Homosporous fern and lycophyte genomes will allow us to reconstruct the ancestral genome structures of seed plants, euphyllophytes, and all tracheophytes and will help us to understand the numerous transitions and key innovations that have evolved during the history of these clades.
Synangiate pollen organs terminate isotomously divided fertile branches and consist of three to six basally fused elongate microsporangia. The male gametophyte releases the male gametes and these reach the female gametophyte to fuse with the egg. Heterospory was evolved from isospory independently by several plant groups in the Devonian period as part of the process of evolution of the timing of sex differentiation. It protruded out above the multilobed integument. The gametophytic generation is a weak link between the sporophyte and seed in the life cycle of terrestrial plants.
To understand the underlying causes of this highly iterative evolutionary pattern, Bateman and DiMichele 1994b dissected heterospory and the seed habit into 12 phenomena that were more discrete and definable, and then sought repetitive patterns among the putative phylogenetic lineages. This reduced the chance of genetic variability among homosporous plants. In a study of megaspore development in Œnothera rubricalyx Gates and Sheffield , 1 it was incidentally discovered that this is not the case. In this review we highlight two ideas that may lead us to a better understanding of why heterospory evolved. These vascular tissues, primarily living xylem, are called sapwood; they conduct water and dissolved minerals from the soil to the leaves. There is reduction to one megaspore in some species, e. Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia.
The megagametophyte of seed plants is retained and nourished by the parent plant within the ovule. These revealed that Selaginella originated on Euramerica around 383 Ma in the Devonian period, while its peak diversification began with the formation of Pangea. Heterospory Advanced to the Seed Habitat in Land Plants: One of the most important advantages of heterospory is the formation of seed habitat. Attempts to macerate the megaspore from the sporangium have been unsuccessful, with only small fragments released during acid treatment. It was an addition to a sexual life cycle that already involved male and female gametes. The microspore germinates to form the male gametophyte and the megaspore germinates to form the female gametophyte. The spongy wall ultrastructure of ProlObarilwphylon pel1l1.
Michael Krings, in , 2009 L ycopsid H eterospory The lycopsids encompass a variety of reproductive strategies ranging from homospory to heterospory, and in some cases evolved reproductive structures that in many ways were functionally similar to seeds Bateman, 1996b. The oldest seed-bearing seed fem Middle Devonian, 385 M had a small, radially symmetrical, integumented megasporangium. The fossil record provides no evidence as to whether megaspore retention preceded or followed the other important changes in the evolution of the seed, such as the reduction to a single functional megaspore per sporangium, the modification of the distal end of the nucellus for pollen capture, and the evolution of integuments and accessory structures such as a cupule. In this context, they may have functioned in a manner similar to that of sporangial elaters in certain bryophytes. Production of two types of spores 2. Such lags in diversification are not uncommon in the fossil record. Homoplastic character-states can be as informative about evolutionary mechanisms as synapomorphies—despite being less valuable for determining relationships— thereby erecting monophyletic classifications.