Botany – Stems

I. External Features of a Woody Twig

A. Node –

1. A _______________________________consists of a blade and a

petiole.

a. The blade –

b. The petiole –

2. Leaf Scars –

a. Bundle Scars- marks within the leaf scars from the

_______________________________ and

_______________________________of the previous year’s leaf

3. Stipules - small appendages found at the base of the petiole of some

leaves; leaf-like flaps

a. Stipule Scars - marks left on a twig by the previous year’s

stipules

B. Internode –

C. Axil –

D. Leaf primordial - tiny, embryonic leaves produced by the apical meristem

E. Bud - protective structure that contains an apical meristem and leaf primordial. Buds

are usually covered by one or more bud scales, which fall off when tissue growth begins.

1. Axillary bud –

a. Responsible for the production of

_______________________________branches.

2. _______________________________- bud at the top of a stem or

branch. They are usually slightly longer than axillary buds.

a. Responsible for _______________________________

3. _______________________________- marks left on the twig from the

previous year’s terminal bud scales

II. Stem Development

A. Primary growth in stems is similar to that in roots. The apical meristem cells

divide and produce the three primary meristems & leaf primordia:

1. Protoderm - found on the outside

2. Procambium - inside the protoderm

a. produces _______________________________and

_______________________________ (the primary tissues)

3. Ground Meristem - forms inside and outside of the procambium

4. Leaf primordia - become leaves

B. Within the stem you will find…

1. Pith –

2. Cortex –

3. Leaf/bud traces –

4. Leaf/bud gaps - small gap in the conductive tissue above each

leaf/bud trace.

III. Stem Tissue patterns

A. The tissues within the stems of primitive plants, gymnosperms, monocots,

And dicots are arranged differently.

B. _______________________________- the central cylinder of the stem;

composed of primarily xylem, phloem, and Pith.

C. _______________________________- primary growth only (These plants are

usually annuals & therefore have no need to produce secondary growth.)

1. Outermost layer is _______________________________, which is

covered by a _______________________________to prevent water loss

2. Vascular bundles composed of xylem and phloem are found inside the

epidermis and surrounded by parenchyma

a. In dicots, the vascular bundles are

_______________________________with pith on the inside and

cortex on the outside.

b. In monocots, the vascular bundles are

_______________________________ throughout the parenchyma

with no defined cortex or pith. Therefore, this tissue is called

_______________________________ (or

_______________________________) tissue.

D. Woody angiosperm stems- originate with primary growth and then continue with

secondary growth

1. In _______________________________, new xylem and phloem are

produced each year by meristem tissue called the

_______________________________.

a. The vascular cambium is initially found between the xylem and

the phloem of the vascular bundles.

b. Each year, the vascular cambium produces new

_______________________________cells to the inside and new

_______________________________ cells to the outside.

c. The previous year’s phloem is killed by the next year’s growth.

2. _______________________________ (or phelloderm) is another type

of meristem tissue found beneath the epidermis

a. After the first year, the epidermis is disrupted by the growth of

the cork cambium and is replaced by

_______________________________ (cork cells and old phloem

cells).

b. cork cells contain a fatty substance called

_______________________________ which makes them waterproof

c. Gas exchange only occurs in pockets of loosely arranged

parenchyma cells called _______________________________.

The cells of the lenticels are not impregnated with suberin.

3. Types of Woody Angiosperms

a. _______________________________- woody dicot trees;

xylem contains parenchyma, fibers, vessel elements, and tracheids

b. _______________________________- coniferous trees; xylem

contains parenchyma and tracheids only

4. Within woody angiosperm stems you may find…

a. _______________________________- patterns produced by

differences in size of xylem cells over the growing season.

1) _______________________________- larger cells,

more vessel elements than tracheids (Trees usually have

more _______________________________ available

during the spring, and thus grow more rapidly.)

2) _______________________________- smaller cells,

more tracheids than vessel elements (Again, this has to do

with water availability. During the dog days of summer,

there is less water available, and the trees grow more

slowly.)

b. _______________________________- extensions of plasma

membranes of ray parenchyma that balloon into vessels and

tracheids. When the tylosis completely fills the conductive cell, the

conductive cell no longer functions.

c. _______________________________- old xylem at the middle

of a tree. Filled with _______________________________ and

accumulated resins, gums, and _______________________________—

very resistant to parasites and rot-causing microbes.

d. _______________________________- functioning xylem near

the vascular cambium

e. Resin Canals- tubes scattered throughout the tissues of many

_______________________________. Lined with resin-secreting

cells. Resin may help ward off _______________________________.

It also seals injuries to the tree.

f. Laticifers- specialized cells and ducts that secrete latex. Cells

remain living unlike vessels. Found in about 20 families of plants.

Latex is a thick fluid composed of gums and other substances.

Function appears to be similar to that of resins.

IV. Specialized Stems

A. Rhizomes – 1. Adventitious roots may develop from the rhizome Photo source: Little House in the Suburbs: How to Plant Bearded Irises (https://littlehouseinthesuburbs.com/2013/09/how-to-plant-bearded-irises.html)

B. Stolons – Usually used for propagation of the plant. 1. Adventitious roots may also develop. 2. Common in _____________________________________ Photo source: modified form shutterstock.com 751255993

C. __________________________________ - underground stem composed primarily of starch-filled parenchyma cells that form at the tips of stolons or rhizomes 1. Usually used for food storage 2. Potatoes are tubers a. the eyes of the potatoes are nodes Photo source: https://www.online-sciences.com/the-living-organisms/the-asexual-reproduction-by-tubers-in-plants/

D. Bulbs- modified stem in which starch accumulates in thickened, fleshy leaves attached to the stem 1. Bulbs are large buds with a very small stem at the bottom. 2. Onions are bulbs a. The layers of the onion are actually non- photosynthetic leaves

E. _______________________________- similar to bulbs, but consist almost entirely of stem tissue 1. _______________________________ and _______________________________are corms Photo source: http://www.corunna-mi.gov/departments/public_works/what_the_hecks_a_corm.php#.XYtxUEZKjcs

F. Cladophylls- 1. Most asparagus “leaves” are actually cladophylls 2. Prickly pears “leaves” are cladophylls Photo source: https://www.freepik.com/free-photos-vectors/cladophyll

G. Thorns- hardened, modified stems.

1. Many thorn-like objects are also modified leaves or stipules

H. Tendrils- stems modified for grasping objects 1. Common in Ivies…other ivies use modified leaves or adventitious roots 2. _______________________________- growth stimulated by touch. Photo source: White pumpkin vine tendril (Kate Russell) from https://www.thedailygarden.us/garden-word-of-the-day/tendrils