Chapter 3. Plants invade the land - the Bryophytes

 

Plants would eventually clothe the Earth with forests, grasslands, and meadows, but at first they were little more than green algae trying to survive outside of the water. They couldn't get very big for two reasons. One, they were still dependent on water for sperm cells to swim from one plant to another to reach eggs for fertilization. This could only occur in films of water in the soil, other flat substrate, or in the spongy matrix of closely packed leafy shoots. .So the soil or other substrate had to be wet, at least part of the year. Even so, sperm cells could travel only short distances, greatly limiting the opportunity for genetic exchange in a population. For that reason, a major new phase of reproduction was added - spores, produced by a small sporophyte, which could be transported through dry air to a new location.  Second, without roots or vascular tissues, they were dependent on absorbing water directly through their vegetative tissues. So they could live only where there was plenty of rainfall, or else be able to survive prolonged periods of desiccation. These non-vascular plants or bryophytes were the amphibians of the plant world. Their vegetative limitations would only be lifted by the evolution of roots and water-conducting tissues in the early vascular plants, but the reproductive limitation would persist until the evolution of seeds in ancient gymnosperms.

Figure 3.1. Non-vascular plants, particularly mosses, continue to thrive today in damp 

 locations. In the Hoh Valley temperate rain forest in Washington State, every 

available surface is covered with mosses. 

 

Figure 3.2. Mosses typically have leafy gametophytes packed closely together to hold waster 

between them like a sponge, while the sporophytes are leafless stalks with elevated sporangia 

exposed for wind dispersal. A. Polytrichum commune, B. Bryum caespiticum, 

C. Hylocomium splendens, D. Sphagnum palustre. Drawings from Kerner and Oliver 1895.

 

 

Figure 3.3. Less common than the mosses with their elongate stems and spirally arranged leaves, 

are thallose liverworts (A), leafy liverworts (B) with leaves typically on two sides of 

flattened shoots, and thallose hornworts that are also thallose.  

Drawings from Coulter et al.1910.

 

 

 

Figure 3.4. The flat, thallose growth form, featuring 2-dimenisonal growth in circumference, 

hugs the ground for enhanced water absorption and retention. It can be found in some green 

algae, like Coleochaete (A), in the thallose liverworts (B), in hornworts (C), and in the

 gametophytes of some vascular plants, like ferns (D). Note the three latter terrestrial plants 

have root-like rhizoids to aid in absorbing water from the soil. Rhizoids are simple 

extensions form individual cells.  Drawings from Coulter et al. 1910.

 

 

Figure 3.5 Sphagnum mosses have a unique architecture with specialized water-storage

 cells. The photosynthetic cells are narrow and form a network running through the water 

storage  matrix. Drawings from Smith 1935. 

  

Figure 3.10. Gametangia in bryophytes are produced by the haploid gametophytes. 

A typical moss antheridium (A) consists of a jacket of protective cells and 

hundreds of flagellate sperm cells little different from unicellular green algae like 

Chlmydomonas. The typical archegonium (B) houses a single egg in a vase-shaped chamber, 

with a long, narrow neck. After fertilization by a sperm cell that has swum through the 

narrow opening of the archegonium, the egg begins to develop into the multicellular 

diploid sporophyte. Drawings from Brown 1935.

Figure 3.11. In bryophytes, the leafless, unbranched, and short lived.sporophyte

 develops from the fertilized egg on the parent gametophyte.  In most mosses (A), as well as 

in most liverworts, the sporangium is elevated by elongation of a distinct stalk that is part 

of the sporophyte,  In Sphagnum (B), the stalk develops from part of the gametophyte 

instead of from part of the sporophyte. In hornworts (C), there is no stalk, but the 

sporangium itself elongates through basal intercalary growth (cell division at the

 bottom, with the oldest tissues at the top).  

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

Figure 3.12.The sporophyte of Riccia consists of a sporangium that remains embedded 

within the thallose gametophyte. Drawing from Brown 1935.

Figure 3.13. The fossil Aglaophyton major was a diploid sporophyte plant with a branching 

pattern similar to early vascular plants, but lacked vascular tissue.  The origin of the 

latter is still a mystery, but this plant may have been a link. 

Drawing after Kidston and Lang 1921.