Lecture 26

Embryo & Seedling

I. Mature Embryo

A.  Monocots and dicots. Number of cotyledons – two for dicots, one for monocots (Figure 24.1). A convenient category for comparisons within angiosperms, but not the “two main taxa” as stated by Esau.  Newer classifications such as APG (angiosperm phylogeny group) frequently used today.  Peter Stevens’ Angiosperm Phylogeny web site is a continuously updated summary of information on the evolutionary relationships among the flowering plants. The phylogenetic tree presented on this site shows that the angiosperms are not split into two groups (monocots and dicots).

B. How monocots evolved one cotyledon was considered controversial at the time of Esau. Is the cotyledon lateral or the shoot apex lateral? Stevens’ web page does not resolve this stating “There has been much discussion about the evolution of the single cotyledon that characterizes the clade - by connation, or by suppression (see e.g. Haines & Lye 1979; Burger 1998)?”  The book “Phylogeny and evolution of angiosperms by Soltis et al. (2005) also does not mention any data that have resolved this issue.

C. Variation in embryo morphology (Figure 24.2; see also Figure 23.2). Hypocotyl length, cotyledon size and shape, etc. The embryos already have procambium, protoderm, ground tissue.

D.  Grass embryos.  Figure 24.3 for Triticum (wheat) and Zea (corn, maize).  The cotyledon is called the scutellum. It is pressed tight against the endosperm.  The embryo axis appears laterally attached to the scutellum.  The radicle is enveloped by the the coleorhiza.  The shoot apical meristem is enclosed by the coleoptile.  These attach at the scutellar node. There is no differentiation between a hypocotyl and radicle.  The procambial system connects the shoot and the root (Figure 24.4).  The scutellum absorbs nutrients from the endosperm and is also a source of enzymes produced during germination.  The coleoptile is a tube with the shoot inside. It reacts to light (positively phototropic) and protects the shoot as it’s growing up through the soil.

II.  Development of the Embryo

A.  Capsella bursa-pastoris (shepherd’s purse – Brassicaceae - images)
1.  The egg and zygote already have polarity established. The chalzal end is more mitotically active than the micropylar end. 
2.  The first cell division establishes a horizontal wall.  Later divisions are both horizontal and vertical. Before one can differentiate a clear embryo and suspensor, the structure is called the proembryo.  The first stages where only an apical tier of cells exists is shown in Figure 24.5A.
3.  Early on, a protoderm and hypophysis form (Figure 25.5B). Then procambium (Figure 24.5C) and derivatives of the hypophysis are produced – this is the globular embryo stage. The hypophysis will eventually produce the root cap.
4.  The embryo begins to differentiate cotyledons (Figure 24.5D) and switches from an axial system to a bilateral organization. The embryo is now in the heart shaped stage.
5.  Continued development of the embryo takes it to the torpedo stage (Figure 24.5D).  More growth and the embryo bends (image) and reaches the chalazal end of the ovule (Figure 24.6B). The apical meristem (of the epicotyl) is now mound shaped and the procambium has differentiated from the cotyledons to the radicle.  At this stage, in the endosperm the free nuclei are becoming cellular and the nucellus is being digested. The endosperm is greatly reduced in size as it is being converted to embryo tissue (Figure 24.6 B to D; image)
6.  The basal cell and lower chalazal suspensor cells fuse with the embryo sac wall. The basal cell enlarges, crushing the integument. The suspensor cells are eventually crushed, whereas the basal cell survives longer (Figure 24.6 B to D). 

B. Allium (onion – Alliaceae).  Figure 24.7. The embryo is at first spherical, then it elongates and forms a lateral notch.  The notch develops into the epicotyl and sheath of the cotyledon (Figure 24.8).  Later the shoot apical meristem forms, as well as the hypocotyl.  Further development is shown in Figure 24.15.  The first leaf pushes through a slit in the cotyledon sheath.

C.  Grasses  Figure 24.9.  Similar sequence as was seen in Allium.  There was apparently disagreement whether the embryo tissues (scutellum, embryo axis, suspensor, etc.) can be traced to tiers of cells (Figure 24.9J).  A similar sequence (of photos) is shown in Figure 24.10 for Zea.  Details of the ultrastructure of early stages in barley (Hordeum) shown in Figure 24.11 and Figure 24.12.

 D. Deviations
1.  Parasitic plants.  Some lack true ovules (e.g. some Santalales). In dwarf mistletoe (Arceuthobium) the female flowers are small and produce a pollination droplet. Here is a diagram of a female flower in L.S. Note there is no true ovule. In Balanophoraceae the female flowers are VERY SMALL such as this Balanophora fungosa.
2.  Mycoheterotrophs (not saprophytes as stated in Esau). Page on Parasitic Plant Connection.
3.  Apomixis – a form of asexual reproduction. In agamospermy, an embryo not produced from a fertilized egg (zygote) but from ovular cells that are already diploid.
4.  Polyembryony – multiple embryos produced in one seed.
5.  Embryoids – embryos produced from tissue culture of plant cells.

III.  Classification of Embryos

A. Based upon variations in the fates of cells formed from the initial divisions of the zygote.

B.  Figure 24.13.  Shading indicates the relative amount of cellular material involved in forming the embryo as opposed to suspensor tissue. The top row also shows the direction of the first wall formed from the apical cell. This scheme was abandoned and simplified – that new scheme is in the bottom row. [this is a bit dated and esoteric. Have these patterns been examined in a phylogenetic context?]

IV.  Seedling

A.  Seedling development is really a resumption of embryonic growth following dormancy.  Cotyledons that were previously storage organs are now photosynthetically active.

B.  Patterns of germination. Photo showing Pisum (pea) and Phaseolus (bean)
1.  Hypogeous.  Seeds leave the cotyledons in the soil upon germination. Examples: Pisum, Zea.
2.  Epigeous.  Cotyledons are elevated above the soil upon germination. Examples: Phaseolus (image), Linum and many other dicots.

C.  Seedling development in Linum (Figure 24.14).  Hypocotyl forms a hook, upon straightening, pulls the seed coat with cotyledons out of the soil. Seed coat is then shed, the cotyledons expand.  Radicle continues growth from the hypocotyl. The epicotyl expands and produces the shoot system.

D. Seedling development in Allium (Figure 24.15). Hypogeous.

E. Seedling development in Asparagus (Figure 24.16). Hypogeous.

F. Seedling development in Poaceae (Figure 24.17). Hypogeous

G.  The transition of tissues from the root to the stem 
1.  As shown in Figure 24.14E, the xylem in the root is exarch, which means the oldest xylem (protoxylem) is in the center. The development (young to old) is from the outside to the inside (centripetal). 
2. This whole arrangement is reversed when we look at the stem xylem which is endarch.  Here the oldest xylem (protoxylem) is to the outside of the axis. The development (young to old) is from the inside to the outside (centrifugal). So how does the anatomy change as you go from root to shoot?
3.  Figure 24.18 shows what happens to the xylem (and phloem) along the root to shoot axis.
Last updated: 14-Oct-22 / dln