OEB 59 – Plants and Human Affairs Lab 7: Wood (2)

Part I: Monocot "wood" and spooky faces

Secondary growth Remember from the first lab (Part I: Recognizing monocots and dicots) that monocots (and some dicots) have lost the ability to increase their diameter through secondary growth, and so do not produce wood. However, some monocots can produce a substitute, as in the palms and agaves.

How to make a monocot tree? Monocots do not have vascular cambia, but many of these have tree-like forms. Palms, bamboos, cabbage trees, and dragon trees are all monocots. How do they do it? Well, in various ways. Plants of the genus Musa (banana, family Musaceae) build up a strong "pseudostem" structure by forming a thick layer of fibrous leaf bases. The trunk is not really woody, but is a rosette of overlapping, tightly packed leaf bases, arising from an underground stem (rhizome). In palms (family Arecaceae), such as the coconut palm tree, there is a "thickening meristem" that adds girth at the base, and then the cells elongate up. Each vascular bundle, which is clearly visible in the cross-section of a palm stem, adds new cells secondarily. However, there is no organized "ring" of secondary vascular tissues as is the case in dicots. The cabbage palm (Cordyline) has a strange lateral meristem like the vascular Cocoa nut palm (Cocos comosa) stem with xylem vessel "eyes" in vascular bundle spooky "faces". cambium of dicots, except that it produces Photo made with difference interference contract whole vascular bundles (xylem and phloem) optics (DIC). (c) David Maitland, UK. towards the inside of the stem.

TO DO: We have a piece of a Cocos stem. Look at it under the stereomicroscope. Additionally, we made some slides of transverse sections. Take a look at one of these slides and draw a vascular bundle (+ label with sclerenchyma, vessels, and phloem). For both (macroscopic and microscopic), check out the enormous vascular bundles in the palms.

Part II: Some weird wood

Winter's bark Drimys winteri is an odd plant, which was once used to prevent scurvy. It is an angiosperm, but its wood is strange: the conducting tissue of the xylem contains only tracheids. As in a conifer! It is therefore considered a "primitive" angiosperm, but it is unclear whether this arrangement is primitive or secondary; the ancestors of the modern Winteraceae (the family to which Drimys winteri belongs) may have had vessels, which may subsequently have been lost. The leaves of Drimys species have a distinctly peppery scent when crushed (but the bark is unscented). TO DO: Look at one of the slides we pulled out. Draw and label a transverse section. (Can you find the pits?)

Gnetum Gnetum is a genus of gymnosperms, tropical trees that have confused many botanists! It looks like a dicot (leaves with reticulate venation, vascular cambium, long open conducting elements in the xylem that look suspiciously porous), but it has "naked" seeds, which makes it a gymnosperm. Interestingly, some species of Gnetum have been proposed to be the first insect-pollinated plants as their fossils occur in association with the extinct pollinating scorpionflies (Mecoptera). [For pictures of Gnetum [and other plants], browse this website: http://botit.botany.wisc.edu/Resources/Botany/.]

TO DO: Take a look at the wood of Gnetum. Does it have vessels? Look at the transverse and radial sections. Draw and label one of these.

Bignonias The Bignoniaceae family is composed of mainly trees and vines, with a few shrubs and herbs. They are angiosperms. Many of these have a strange feature, namely a cross or other star-shaped patterns of enormous rays of phloem that are clearly visible in transverse sections of the stem. The origin and function of these is unclear. Researchers have speculated that their thin walls give the vines added flexibility. Alternately, since many phloem cells (unlike xylem cells) are living, it may be that these inclusions give the stem extra regenerative properties when damaged. See the phylogenetic tree to the right from Pace et al. 2015 (Plant Syst. Evol. 301/3), where the six types of "cambial variants" found in Bignoniaceae are mapped onto. You can easily see how the phloem makes arcs or few to many wedges. Many Bignoniaceae are tropical trees. Contrary to trees in the temperate zone, trees in tropical regions grow year round and therefore show no real obvious annual growth rings.

TO DO: Draw and label the transverse section of two examples. Looking at Pace et al. 2015, which type (1 to 6) do you think you have drawn?

The genus Catalpa belongs to the tribe Catalpeae within the Bignoniaceae family. You observed this tree during the walk earlier today (at both sides of the gate of Harvard Yard looking out on the Science Center). Catalpa was once used as an ornamental street tree, but its brittle wood and leaf, twig, and fruit litter has made it less popular nowadays. The long-shaped fruit is a capsule (= a dehiscent dry fruit that splits open in a variety of ways).

TO DO: Observe the transverse section of Catalpa. Does it have ring- or diffuse-porous wood?

For more information about wood (including lots of pictures), see this website: http://legacy.ncsu.edu/WPS202/syllabus.html

Prepared Danny Haelewaters 2015