Chapter 4. Vascular tissues and the Seedless Vascular Plants

  The major adaptation of vascular plants was the invention of a system of tubes for transporting water

(xylem) and dissolved food (phloem) throughout the plant. These together are the vascular tissues. This occurred in the sporophyte generation, which could then get taller, enabling greater competition for light and higher launching of spores into the wind. The evolution of roots from downward-growing stems provided sturdier anchoring and the tapping of deeper sources of water and nutrients. They formed the Earth's first forests.

The first vascular plants, however, saw little advancement in the gametophyte generation, and were still dependent on external films of water for swimming sperm cells. Only with the evolution of seeds and pollen grains would plants be freed from this constraint, as you'll see in the next chapter. 

Pictured on the right is a simple tracheid, the primary cell type of xylem tissue. Tracheids are highly elongate cells with thick, sturdy walls that provide support for the upright stems in addition to their water-conducting function. Tracheids overlap with one another, forming bundles that connect with all plant organs. The side walls contain numerous pores, which have thin membranes that allow water to flow from one tracheid to another. 

 

Figure 4.1. A swamp forest in the Carboniferous Period, some 300 million years ago, 

consisting of giant forms of seedless vascular plants like horsetails, club mosses, and ferns.

 Such forests were extensive at this time, and were the basis of much of our coal deposits. 

Hence the name for this period. Animal life at this time was dominated by amphibians 

and insects. 

 

 

 

Figure 4.2 The first vascular plants, like this Horneophyton lignieri, were simple forking 

stems, bearing sporangia at their tips. Rhizoids sprouted from the basal rhizome.

 Drawing from Brown 1935.

Figure 4.3. Early vascular plants soon evolved ways to increase the number of sporangia by

 positioning them on very short lateral branches, as in Zosterophyllum (A) or by branching 

the stem tips several times to produce sporangia, as in Psilophyton (B). Zosterophyllum is 

considered ancestral to modern club mosses, while Psilophyton developed coiling of the 

young stem tips, which would be passed on to its descendants, the  ferns.

 Drawings from Smith 1935. 

Figure 4.4. Beginning with simple forking stems (A), leaves evolved in two major ways, 

defining the major groups of vascular plants. In the ancestors of club mosses, simple 

leaves (microphylls) evolved from outgrowths of the surface tissues (B). In the ancestors 

of ferns, horsetails, and seed plants, forking branch systems were displaced to the side of 

the main shoot and webbing developed between them (C), forming what we call  

megaphylls.  The vascular tissues formed in a central column, typically with the 

phloem (food-conducting tissue) surrounding the xylem (water-conducting tissue).

Drawing from Smith 1938.

Figure 4.5. Roots began as downward growing stems in early vascular plants with the 

vascular tissues remaining in a central column, the phloem surrounding the xylem (X-shaped); 

rhizoids became root hairs, and a root cap evolved to protect the growing point

 (apical meristem) . Redrawn after Raven et. al. 1999.

. 

 

 

 

Figure 4.6. Club mosses (Lycophyta) come in many forms, but are characterized by 

microphyllous leaves and strobili consisting of numerous sporangia lined up along specialized s

tems. A. Huperzia phlegmaria, an epiphyte with downward facing leafy stems and 

branched strobili. B. Lycopodia cernua, a ground-dewlling species with strobili forming at the

 tips of leafy shoots. C. Lycopodiella clavatum, a ground-dwelling species with branching 

strobili atop a long specialized stalk. Drawings from Brown 1935.

 

 

Figure 4.7. Isoetes is a highly specialized lycophyte, with a condensed, bulb-like base 

and elongate, upright microphylls. Strobili are in the axils of the leaf bases. Note the 

presence of true roots, which occur in most lycophytes. Drawing from Haupt 1953.

 

 

Figure 4.8. Ferns are the most abundant and diverse of modern day seedless vascular plants. 

The megaphylls are typically large and branched, developing from coiled buds, and arising 

from rhizomes with true roots (A). Sporangia are borne typically on the lower surface of leaves, 

in clusters covered by a flap of tissue called an indusium (B,C). 

Drawings from Brown 1935 (A), Haupt 1953 (B), and Transeau et al. 1940 (C).

 

Figure 4.8A. The resurrection fern, Pleopeltis polypodioides, is an epiphyte with a 

creeping rhizome that clings to the bark of a tree. Unusually among vascular plants, 

it is capable of being desiccated during dry weather, and reviving after a rain. 

 

 

Figure 4.8B. The tropical epiphytic bird's-nest fern has a short, non-creeping, upright stem, with leaves in

 a circular arrangement called a rosette. Leaves and other debris, as well as rain water,  

accumulate within the rosette, providing nutrients to the plant. 

  

Figure 4.9. One of the measures of success in the ferns is their variety of specialized

 habits. Some have become aquatic, as in this Salvinia. The plants are free-floating rosettes, 

with roots dangling in the water. Their sporangia-bearing leaves have been modified 

into capsule-like structures, called sporocarps, which mature underwater. 

Drawing from Coulter et al. 1910. 

 

Figure 4.10. The horsetails (Sphenophyta), evolved a novel growth pattern, which is 

remarkably like that of modern banboos. An entire shoot develops first as a compact 

bud, with nodes pressed closely together. It can then elongate rapidly through elongation

 of each internode independently. Growth of the internodes occurs via cell division 

in  basal intercalary meristems. Drawing from Kerner and Oliver 1895.

Figure 4.11.  While leaves in modern horsetails are scale-like, fossils of this ancient

 Sphenophyllum, reveal them to be megaphylls (A), and so they are classified with ferns 

and seed plants as Euphyllophytes, which are distinct from the separate lineage of 

microphyllous Lycophytes. The sporangia of Sphenophyllum (B, C) were borne in the

 axilx of strobili bracts that were also megaphyllous in nature. 

Drawings from Smith 1938.

 

 

Figure 4.12. Modern horsetail strobili (cones) (A) lack bracts. Sporangia 

instead are protected underneath shields (B) that evolved from the back side of inverted 

clusters (C) in an ancient ancestor. Spores of modern horsetails have elongate appendages, 

called elaters, (D, E) that can twist and turn with changes in humidity, pushing the 

spores out of the sporangium. 

Drawings from Kerner and Oliver 1895 (A, B, D, E) and Haupt 1953 (C).

 

 

Figure 4.13. The whisk fern, Psilotum (A, B) and its relative Tmesipteris (C), are rootless

 epiphytes, long thought to belong to  a primitive ancient lineage, are now thought to 

be specialized ferns, according to modern DNA analysis. Drawings from Brown 1935.

 

 

Figure 4.15. Tree-like vascular plants have evolved in several ways. In tree ferns, rhizomes 

grow upward and are covered by a mat of roots ("tree fern fiber") that absorb water and 

transmit it up to the leaves. The actual stem is relatively narrow. 

Drawing from Thome 1885.

Figure 4.16.  Lepidodendron (A), Sigillaria (B), and Calamites (C) were 

early trees of the Carboniferous period that produced some limited amounts of 

secondary xylem (wood) from a unifacial cambium, but no secondary phloem. 

Drawings from Smith 1935

Figure 4.18. Archeopteris had well-developed secondary xylem and secondary phloem in 

both leafy branches and roots, forming the first true, long-lived trees (A) resembling

 modern gymnosperms. Sporangia, however, were borne on the underside of leaves (B), 

like ferns, and so are considered "missing links" between the two groups.

 Drawings from Beck 1961.

 

 

Figure 14.19. As plants boldly moved onto drier habitats, another important feature was the 

evolution of stomata (A).  The upward flow of water in the xylem depended on evaporation

 from the top of the plant, but too much evaporation brought the risk of desiccation, so stomata 

evolved to control that evaporation. Each stoma consists of a special apparatus of cells surrounding

 a pore (B). The pair of sausage-shaped  guard cells (B) bow outward to open the pore

 by absorbing water, and collapse together by expelling water to close the pore. 

Drawings from Brown 1935 (A) and Ganong 1916 (B, C).

 

 

Figure 4.20. The vascular cambium of Archaeopteris and seed plants is bifacial, 

which means it produces new xylem tissue to the inside, which build up as layers of wood,  

and new phloem tissues to the outside, which may build up for several years, but are 

eventually sloughed of with the bark. Drawing A from Ganong 1916.

Figure 4.21. Fully developed modern trees have roughly the shape of an hourglass, with

 balanced above- and below-ground woody branching systems. The trunk serves to connect 

the two systems. Through secondary growth, xylem and phloem systems are continuous

 throughout  the whole plant.