Chapter 9. the Monocots

 

The monocots are a unique group of flowering plants with an extraordinary architectural departure from the gymnosperms and other angiosperms. They have a single seedling leaf, or cotyledon,  and underground stems with leaves that arise from them through basal growth. With roughly 80 families, nearly 3000 genera and 70,000 species, their diversity and ecological impact rival that of the dicotyledonous plants, and traditionally were considered a parallel and equal taxonomic subclass.an has led to a diverse and successful, and numerous array of primarily herbaceous perennial plants. 

1.The primary feature of monocots is a creeping underground stem, or rhizome, though in specialized forms the stem may be climbing or erect.

2. The single cotyledon of the monocot embryo has a sheathing base that encircles the stem axis, creating a protective pocket for younger leaves.

3.The fundamental type of leaf in monocots, including the cotyledon, is linear or sword-shaped, generally with a sheath that encircles the stem, and numerous parallel veins entering the sheath independently from the stem.

4. Monocot leaves elongate through cell divisions at the base, in a  basal intercalary meristem.

5. More specialized leaf shapes involve the parallel vascular bundles bowing outward or diverging from a midrib, but the bundles still originate independently from the stem through the leaf sheath.

6.Woody growth was abandoned by the ancestral monocots, and tree-like forms, like palms or bamboos, were reinvented from scratch, relying on fibrous bundles for strength rather than wood.  

Figure 9.1 Grasses are the epitome of monocot evolution, surviving drought, fire, and 

grazing as underground rhizomes, and sprouting new leaves quickly when good growing 

conditions return. Grassed dominate vast areas of open, semi-arid land and support huge 

animal populations, not to mention the cereal grains upon which civilizations have 

depended on for millennia. 

 

Figure 9.2  Monocots grow primarily from horizontal rhizomes, as in this sedge (A) and Iris (B. 

Drawings after Thome 1877 (A) and Gray 1876 (B)

Figure 9.3. In monocot seedlings, the tip of the single cotyledon typically remains within 

the seed to absorb nutrients while the young plant establishes itself. In the seedling on the

 left,  typical of palms, the cotyledon elongates to push the embryo downward into the soil. 

In the onion seedling on the right, the cotyledon emerges from the soil and pushes the 

seed up with it.  Drawing from Kerner and Oliver 1895.

Figure 9.4. The Hippeastrum plant illustrates the typical growth pattern of monocot leaves. 

The older part of the leaf is the tip (colored darkly). New tissues are added at the base, pushing

 the older tissues upward. The overlapping leaf sheaths form an underground bulb. 

Figure 9.5. The leaf sheath encircles the stem at the base, and elongates from a basal intercalar 

meristem, The flatter leaf blade typically elongates from its own basal intercalary meristem. 

Drawing after Transeau et. al. 1940.

Figure 9.6. In this diagram, the newest leaves of the shoot apex are exposed, showing the 

developing leaf sheath encircling the tip of the stem. A ring of vascular bundles enter the 

base of the sheath from the stem, then converge to form the base of the young blade. 

Drawing from Sachs 1874. 

Figure 9.7. Variation in leaf shape comes from bowing the parallel veins to the side. 

In Smilax (E), an almost dicot-like netted venation pattern emerges as smaller veins branch 

off from the widely diverging parallel veins.

 Drawing after Kerner and Oliver 1895 (A-D) and LeMaout and Decaisne 1876 (E)

 

Figure 9.8. The more complex leaves of many aroids, such as this Monstera, show a pattern 

of diverging bundles of veins and development of cross veins. Drawing from Thome 1877.

Figure 9.9. Some monocot leaves are paddle-shaped, with the parallel veins constricted into 

a distinct petiole and midrib, then diverging one-by-one to the sides 

 Drawings from Ganong 1916.

Figure 9.10. The tree-like form of banana plants IA) is completely herbaceous, with 

the  trunk (B) consisting of the overlapping leaf sheaths. Drawings from Brown 1935.

Figure 9.11. The leaves of palms are the largest of any angiosperms, They are highly fibrous,

of pinnate (C) or palmate (D) form, and split into folded segments.The folds begin as parallel

 ripples in the very young leaves (A) 

Drawings modified from  Corner 1966 (A-C) and Ganong 1916 (D)

Figure 9.12.  The vascular bundles of a palm trunk appear to be randomly scattered, but

 begin as a ring where each sheath attaches to the stem (B). The bundles of successive leaves 

enter near the center of the broad shoot apex and are bent to the side as the upper stem 

expands (C), creating the scattered look.  Drawing D from Brown 1935.

Figure 9.14. A corm is an upright, compact stem specialized for food storage. After a 

seasonal upright leafy shoot and flowers have died back, new corms form on top of the old one. 

Drawing from Ganong 1916.

Figure 9.13. Edible ginger "roots" are actually compact rhizomes that bear upright leafy 

shoots and flower stalks. Like bananas, the upright stalks are built from successively 

longer leaf sheaths. Drawing from Brown 1935.

Figure 9.15. Bulbs are underground food-storage organs built up from swollen leaf sheaths 

that remain  after the seasonal upright leaves and/shoots have died back. In true lilies (Lilium), 

on the left, the swollen leaf bases remain separate and incompletely surround the basal stem, 

while in onions (middle and right) they completely surround the internal stem (forming 

"onion rings") and are packed tightly together. The brown papery coverings on onion bulbs 

is derived from the dried up remains  the outer rings. Drawings from Brown 1935.

Figure 9.16. The majority of orchids are tropical epiphytes. Their thick roots are adapted not 

only to cling to bark and branches, but also to absorb and store water, and are often photosynthetic 

as well. Stems and leaves also store and conserve water in this arboreal habitat that has a daily 

wet had dry cycle.  Drawing from Gray 1879.

Figure 9.17. Most bromeliads are also tropical epiphytes, but have a different mechanism 

for storing and absorbing water. The leave overlap tightly to form an open "tank" between 

them. Daily or periodic rain water is stored in the tank, and in many, specialized roots grow into 

the tank to absorb water. In some, like the "Spanish moss", the leaves themselves absorb 

water and can endure periodic desiccation. Drawing modified from Brown 1935.

Figure 9.18. Palms may be colonial (A), forming clumps by branching at the base, or solitary (B), becoming more tree-like. Drawings from LeMaout and Decaisne (A) and Thome 1877 (B)

Figure 9.19. Members of the Pandanus family also may be tree-like. Stems are able to branch 

and increase in thickness as new aerial roots form ever higher on the tree to provide support 

and vascular tissues. The massive aerial roots in the right picture, dwarfing the human subject 

within, belong to a giant rain forest Pandanus in Papua New Guinea. 

Drawing on left from Brown 1935. Photo on right cc. Frederick Essig.

Figure 9.20.  Bamboos are giant grasses with densely fibrous hollow stems. New shoots (right) 

arise from underground rhizomes and elongate rapidly via intercalary meristems in each

 segment.  Drawing (A) from Thome 1877

Figure 9. 21, A few monocots, such as this Dracaena draco in the Canary Islands, 

have reinvented the vascular cambium, but one that produces whole new vascular 

bundles, rather than secondary xylem and phloem like dicots. 

Drawing from Brown 1935.

Figure 9.22. In some monocots of arid climates,such as this Aloe, leaves have been

 modified to store and conserve water. 

Figure 9.23. The tissues in aquatic plants often incorporate large air spaces or canals for 

buoyancy as well as for conducting oxygen down to roots growing in anaerobic mud.  

Drawing from Brown 1935.

Figure 9.24. The floating aquatic monocot, Hydrocharis, is mostly submerged, but has broad 

leaves, resembling those of waterlilies, that float on the surface. Flower buds are forming 

underwater, but will rise to the surface when opening, for pollination by insects.  

Drawing from Kerner and Oliver 1895.

Figure 9.25. The aquatic monocot sea grass,Vallisneria, produces elongate leaves from 

rhizomes growing in the mud below water. As illustrated in Chapter 7, flowers 

rise to the surface for pollination, with the male flowers floating freely. 

Drawing from Kerner and Oliver 1895.

Figure 9.26. Elodea is a submerged aquatic monocot with thin leaves adapted for

 absorption  of carbon dioxide and minerals. It is frequently planted in freshwater aquaria. 

Photo by Christian Fischer Wikimedia

Figure 9.27 A, Sagittaria (Allismataceae) is rooted in the mud and its broad leaves emerge 

through elongation of the leaf stalk. B. Water hyacinth (Eichornia, Pontedariaceae), which can 

become a river-clogging weed when spread outside of its natural environment, is free floating, 

with bulbous leaf bases adapted for flotation. Drawings modified from Shaukat in

 Flora of Pakistan, Missouri Botanical Garden (A), and Brown 1935 (B)

Figure 9.28. Papyrus is a giant sedge (Cyperaceae). Its head of photosynthetic and 

reproductive structures is lifted high above water level by the elongation of a 

single internode via a basal intercalary meristem. 

Figure 9.29. The origin of monocots is obscure due to lack of early fossils. The unusual 

dicot, Eryngium yuccifolium, provides some clues as to possible adaptations of monocot 

ancestors. The plant looks for all the world like a monocot, with linear leaves sporting parallel

 veins, that arise from basal intercalary meristems.  The flowers identify it clearly as a member of the dicot family Apiaceae (carrot family). Its seedling (see next picture), however, is a dead giveaway. 

The seedling of Eryngium yuccifolia has two cotyledons, but immediately above, the shoot 

takes on its monocot form, with a sheath enveloping the stem, and elongate leaves arising 

from basal intercalary meristems.

Figure 9.31. The genus Acorus sits at the end of what appears to be the most ancient lineage 

of monocots, yet possesses some advanced features: a leaf folded in on itself and fused 

together as in iris plants, flowers with carpels fused into compound pistils and highly reduced 

on a specialized aroid-like spadix. Drawings from Barton 1818 (A) and Thome 1885 (B).

Figure 9.32. The genera Tofieldia (A) and Butomus (B) are members of the order Alismatales,

 and retain some archaic features lacking in  Acorus, including carpels that remain distinct from one another and ordinary flat leaves. This illustrates two facts: One, archaic features may be scattered among living species, and, two,  even though a lineage may be ancient, many changes may have occurred in intermediate species along the way resulting in a relatively specialized modern descendant that little resembles the earliest ancestors. Drawings from LeMaout and Decaisne 1876.