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Nodes - Internodes

NodeInternodeColeusLab.jpg (55059 bytes)
The attachment point of each leaf to the stem defines a Node.This term refers to the stem, not the leaf. The length of the stem axis between successive nodes is called an  Internode.

 

Leaves are tightly coiled around the shoot apical meristem. It may be possible to point to individual nodes, but there is little internodal elongation near the SAM.

wpe9.jpg (15975 bytes)
Agave flowering stem. The base of each leaf defines a Node. The length of stem between two nodes is an Internode.

As leaf primordia become displaced from the SAM, internodal elongation begins. BranchSAM240NodeInodcLab.jpg (131290 bytes)

Shoot Apex of Forsythia. Leaves are attached at the Nodes. Some internodal elongation is apparent in this shoot tip. Note the Axillary Buds which also occur at the Nodes.

Elongation is sequential and resemblesBranchSAM240NodeInodeLab.jpg (90535 bytes) an old monocular telescope (spy glass) with the shortest and most slender segments at one end and the longest, widest segments at the opposite end.


Simplifed Model of Internodal Elongation - The level at which each segment meets is a Node. The length of each cylindrical unit is its Internodal Length.TelescopeModel.jpg (16816 bytes)

Stem elongation causes a separation of the leaves along the stem axis.

Internodal growth is determinate, and ceases after a prescribed period.

Elongation can be infludnced by a number of environmental factors including light, wind, nutrition and water availability.

The nodes generally produce axillary buds which can become  branches. Furthermore, root primordia may be present, especially with monocots like bamboo.

BambSpear240Lab.jpg (15821 bytes)

In Bamboo, the leaves are tightly appressed to the stem and encircle it at their base. It is difficult to point out nodes as the leaves also overlap one another near the stem tip.

As the Internodes elongate it becomes possible to BambLeafNode240Lab.jpg (11921 bytes)see the individual Leaves. Each leaf has a gretly reduced green Lamina and an extensive sheath which is attached at the node.

 

This photo shows the disposition of the leaves,BambLeafNodf240Lab.jpg (15158 bytes) nodes and internodes. The leaf from the preceeding image is seen at the top of the right stem. The leaf just node below it is split longitudinally on one side, and  encircles the stem at the node.

The leaves are ephemeral and fall away as the stem matures. This clearly reveals the internodes.BambShrtInode240Lab.jpg (35961 bytes)

There was far less internodal elongation with the adjacent stem. Note the buds which occur at each node.

 

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