Land Plants: challenges from their environment
Plants growing on land experience some major challenges not shared by algae growing in aquatic or marine environments. In water, algae are surrounded by their most basic needs (minerals, dissolved gases, water and the buoyancy that water confers). Access to adequate light can be a challenge for algae especially depending on how deep in the water they occur. Light can be a problem for land plants too, it may be too bright or too shady, again depending upon their location. Generally, most cells in an alga are able to divide indefinitely. It is the angle at which cell division occurs which confers a characteristic shape on an individual algal colony of filament.
In land plants, on the other hand, water must be accessed from the soil. Surface water evaporates so that the plant also needs a firm epidermis to reduce drying. Another important feature is that all land plants exhibit terminal or lateral meristems or centres of continued cell division. Behind the meristem, the newly produced cells mature and stop dividing. For specialized tissues to develop in appropriate locations, signaling is an important difference between land plants and algae. Indeed auxin (a signalling protein) is essential for controlling the vast diversity of land plant shapes, even including the liverworts, mosses and hornworts. Another most important difference between algae and land plants is the remarkable ability of land plants to respond to gravity. In most land plants, the shoot grows up, opposite the effects of gravity, while roots (or simple rhizoids in bryophytes) grow down.
Other than mosses, liverworts and hornworts (bryophytes in general) growing on land, if plants are to grow taller and more elaborate, they need a conducting system. These features include roots with a root cap and root hairs for better absorbing of water and minerals, leaves for gas exchange and the capturing of sunlight, and a water-conducting system made up of xylem cells and phloem for conducting food. For even larger plants they need secondary thickening called wood. The plants with vascular tissue are called tracheophytes and they include all land plants except the bryophytes (mosses, liverworts and hornworts which hug the soil and tend to shelter in shady moist environments). Some tracheophytes alternatively can withstand very arid conditions.
There are thus major challenges which land plants have to meet with particular design features to allow them to survive on land. The tracheophytes exhibit even more particular design features to allow them to survive farther from a source of water in the soil. However, there is yet another major difference between the bryophytes and the tracheophytes which makes serious difficulties for evolution theory. That difference is patterns of alternation of generation. The bryophytes exhibit haploid dominance while the tracheophytes exhibit diploid dominance. How did the tiny dependent sporophyte of bryophytes become the dominant plant generation in tracheophytes?
Critical to the comparison of the two groups of land plants is the fact that many specialists continue to look for intermediate designs which bridge design features between the two groups. For example, Morris, Puttick et al. published a paper in 2018 which discussed seven different lines of descent among bryophytes with reference to vascular plants (tracheophytes). Their contention was that “almost every possible solution” to the question of lineage in land plants is “currently considered viable.” [Jennifer L. Morris, Mark N. Puttick et al. 2018. The timescale of early land plant evolution. Proceedings of the National Academy of Sciences115 #10 E2274-E2283. See p. E2274 last 2 sentences of first paragraph of text. www.pnas.org/cgi/doi/10.1073/pnas.1719588115 ] The take-home lesson from all of this is that there are no obvious evolutionary relationships among plants groups. Separate creation certainly fits our observations better!