Chapter 8. Dicotyledonous Plants

 Angiosperms descended from gymnospermous ancestors, and share with them a basic woody
architecture with secondary growth in branching above-ground leafy systems and below-ground branching root systems. The evolution of flowers and their capacity to reproduce more rapidly, also resulted in a wide variety of herbaceous angiosperms where woody growth has been nearly or completely abandoned. The latter include the unique clade of monocots, which will be the subject of the next chapter. The remaining clades of angiosperms, characterized by seedlings with two cotyledons are referred to informally as dicots. Because the monocots branched off from among them, dicots are thus a paraphyletic assemblage or grade.

1. Dicotyledonous plants almost always begin with bilaterally symmetrical seedlings with two
opposite seedling leaves, or cotyledons, as in most gymnosperms other than the conifers.

2. The seedling root in most dicotyledonous plants persists either as a taproot, or as a branching underground root system.

3. Dicotyledonous plants almost always have leaves with netted, or reticulate, venation, due
to 2-dimensional expansion during development.

4. The leaves of dicots exhibit extraordinary plasticity in shape and function, with some modified as spines, tendrils, insect traps, or underwater photosynthetic organs. 

5. Dicotyledonous plants, particularly in the more advanced clade Eudicots, have flowers with parts in 4's or 5's

 

Figure 8.1. A great many dicotyledonous trees in multiple families are

 woody, broad-leafed trees. Drawing from Kerner and Oliver 1895.

 

Figure 8.2. Dicotyledonous seedlings are bilaterally symmetrical, with two opposite

cotyledons. In most, the seedling root persists either as a taproot, or as a branching underground 

root system with secondary growth. It is likely that the first angiosperms had two cotyledons, 

as do most gymnosperms other than conifers. Drawing from Kerner and Oliver 1895

Figure 8.3. The primary vascular system of dicotyledonous plants consists of a ring of vascular 

bundles with phloem to the outside and xylem to the inside. If the plants become woody with 

age, the vascular cambium forms between the xylem and phloem and begins to produce 

secondary vascular tissues.

Figure 8.4. In the secondary xylem of woody dicotyledonous plants, there is often a 

division of labor between larger vessels, specialized for conducting water, and very narrow

 fibers, specialized for providing strength and density to the wood. In this cross-section, we see 

the transition from the denser wood laid down in the Fall, and the lighter wood, containing 

large vessels, laid down in the Spring

Figure 8.5. In the rosette growth form, the central vertical stem remains compressed 

and the leaves spread out in a circular arrangement, as in the common dandelion (A) 

and the ever-fascinating venus flytrap )B). This growth form is common in open sunny

locations. In the dandelion, we can see the long taproot, which is a permanent part of the

plant's central axis in many dicots and gymnosperms, but not in seedless vascular plants or 

monocots, which we will see later. Drawings from Ganong 1916 (A) and Brown 1935 (B)

Figure 8.6. Bushy, weakly woody herbs, like this sunflower are common among dicots.

Figure 8.7. Banyan trees and other members of the fig genus, Ficus, have the unusual,
but highly successful habit of extending adventitious roots (roots arising from stems or
leaves, rather than branching from other roots) to the ground from horizontal branches.
This forms a colony that can extend the footprint of the tree for several acres.
Drawing from Gray 1879.

Figure 8.8. Some herbaceous dicots, like this Saxifraga flagillaris, are able to form colonies
by forming new plantlets at the ends of long runners. An even more familiar example of this
habit is the common strawberry. Drawing modified from Kerner and Oliver 1895.

Figure 8.9. The growth forms of dicotyledonous plants are highly varied and flexible due
to the careful balance of plant hormones. Ordinary cabbages can be converted into spindly
elongate plants by the artificial application of the hormone gibberellin. Photo is of
Professor Sylvan Witwer, a pioneering researcher in the field of plant growth regulators,
courtesy of Mrs. Witwer.

Figure 8.10. Herbaceous dicots, like this Anemone nemerosum (Ranunculaceae) may
also spread horizontally via rhizomes, much in the pattern of more ancient seedless vascular
plants like ferns. We will see in the next chapter that this is the primary growth form
of monocots. Drawing from Kerner and Oliver 1895.

Figure 8.11. Some dicots, like this Dahlia, store nutrients in swollen tuberous roots for

next season's growth. buds for new growth can be seen here forming at the base of the 

previous year's stem. Drawing from Brown 1935.

Figure 8.12. Vines are a common growth form among dicots, having evolved via two

distinct kinds of adaptations. In Humulus (A), the main stem is sensitive to touch

(thigmotropic), curving when it senses a solid support. In Bryonia (B), the touch-sensitive

organs are tendrils, thread-shaped structures derived from modified leaves, leaf parts, stipules,

or stems. Drawings from Kerner and Oliver 1895.

Figure 8.13. Dicot leaves typically form first in miniature, then expand via diffuse
2-dimensional growth of their tissues until reaching their full size. As they expand, the
earliest veins branch, forming a mesh of ever smaller veins sufficient to supply all cells
with water

Figure 8.14. The result of leaf expansion, with ever finer branching of veins, is this netted
pattern, or reticulate venation. Drawing from Coulter et al. 1910.

Figure 8.15. The shapes of dicot leaves is highly varied, with blades simple (A) lobed (B,D),
pinnately compound (C), or palmately compound (E). Drawings from Ganong 1916.

Figure 8.16. An unusual growth form among dicots is the palm-like or tree-fern-like
single trunk with large compound leaves produced from a massive terminal bud,
as in this papaya plant.

Figure 8.17. Many plants protect themselves with sharp appendages, which have arisen in
different ways. A. Prickles, as in the rose family, are outgrowths from the surface of a stem.
B. True thorns are modified stems. Spines are modified leaves (C), or parts of leaves (D)

Figure 8.18. Cactus-like succulents have evolved in a number of families, demonstrating convergent evolution.

Figure 8.19. Insectivorous pitcher plants have evolved in different families, including the
American Sarraceniaceae (A) and the old world Nepenthaceae (B). Insects are lured to
these traps, which are highly modified leaves, by colors and/or odors, and sink into interior
pools where they are digested. Carnivorous plants can survive in nutrient poor soils,
swamps, or on tree branches.
Drawing from Ganong 1916 (A), and LeMaout and Decaisne (1876) (B)

Figure 8.20. Some genera exude sticky substances onto their leaves that attract, trap, and
digest their insect prey. The genus Drosera (A) contains hundreds of species found all
over the world, with leaves bearing  specialized glandular hairs. Pinguicula (B) is simpler with a sticky
leaf surface. Drawings from Brown 1935 and Coulter (A) and LeMaout and Decaisne 1876.

Figure 8.21. The bladderwort, Utricularia, is an aquatic plant with intricate traps that suck
in tiny animals that come close enough to brush against trigger hairs.
Drawings from Brown 1935.

Figure 8.22. Convergent evolution has led to lookalikes in different families.
Nymphaea (Nymphaaeceae), Nymphoides (Menyanthaceae), and Nelumbo
(Nelumbonaceae), all send up "lily pad" like leaves from underwater rhizomes,
but their flowers and fruit are different in structure.
Drawings from Masclef 1891 (A), Sturm 1796 (B) and Kerner and Oliver 1895 (C).

Figure 8.23. Aquatic dicots, like Cabomba (A), Ranunculus aquatica (B), and 
Myriophyllum (Haloragaceae), have highly dissected leaves on their underwater stems.
Water flows freely around these narrow leaf segments, providing greater access to
minerals and carbon dioxide. Drawings from LeMaut and Deaisne 1879 (A),
Thome 1885, and Die Gartenlaube 1887 (C).