Two groups of tissues, the xylem and phloem, are important for the movement of herbicides, water, nutrients, sugars, and naturally occurring growth regulators in plants. Xylem tissue moves water and nutrients from the roots to the leaves. Typically soil-applied herbicides move upward in this tissue. Phloem transports manufactured plant food (sugars) from the leaves to points of active growth, including root tips, reproductive tissues and storage organs. In grasses and broadleaf forbs, these two tissue types are grouped together into vascular bundles, which are evident as veins (Figure 10).

In woody plants, the inner and outer bark is composed of phloem tissue (Figure 11). The wood, sapwood and heartwood, is xylem tissue. They are separated by the cambium, the living tissue responsible for tree growth. This is the part of the tree that should be treated during a fresh cut stump application. Girdling a tree by removing a ring of bark from around the trunk prevents plant food from reaching the roots. The major portion of the wood in a tree provides structural support. Living tissues are on the outer edge of the wood and on the inner edge of the bark. The rest of the bark is nonliving tissue that protects the tree from injury.

Some herbicides move only in the xylem tissue and others are translocated in both the xylem and phloem. Most soil-applied herbicides primarily move in the xylem. In contrast, most foliar-applied herbicides move primarily in the phloem. The product label will indicate where, when, and how the herbicide is to be applied to place the material in the best location for plant uptake and maximum control.

Water carries dissolved nutrients up to the leaves and sugars (sap) to the roots. Nutrients in the soil and soil-applied herbicides must be dissolved in water and taken up by the roots of the plant. Water is also a necessary part of photosynthesis -- the manufacturing of sugar from water and carbon dioxide in the presence of green chlorophyll with sunlight as the energy source. The effects of herbicides are reduced under drought conditions, as is photosynthesis.

During drought periods, plants undergo growth stress conditions and produce thick waxy layers on the leaves. They may close their stomates (small pores in the leaf surface) and reduce their metabolic rate to protect against excessive moisture loss. Because photosynthesis and, consequently sugar production, are slowed, translocation of nutrients and herbicides will also be greatly reduced. Under these conditions, herbicides do not penetrate the leaf surfaces or move through the plant easily, so control may be reduced and their use is questionable. Plants growing in dry (arid) environments adapt to these conditions; for example, they may have small, thick leathery leaves; a thick waxy layer on the leaf surface; or the ability to grow and flower rapidly when rain occurs.

Temperature influences all plant activities--absorption of water, transpiration (the evaporation of water from plants), respiration (plant food is used to produce energy), germination, growth, and reproduction. Temperature is one of the most important environmental factors affecting evaporation. Evaporation cools the leaf so its temperature is not as high as the surrounding air. Plant growth tends to increase when temperature rises, and it declines when temperature falls. Most plant growth occurs between 50-100o F (10-38o C). Temperature is also a factor in determining how far north and south a specific plant will grow because temperature extremes determine the length of the growing season or frost-free period.

At high relative humidity, plant leaves are more succulent, may have less of a waxy layer, and a thinner cuticle. In addition, the cuticle accumulates water. As relative humidity increases, transpiration decreases. High relative humidity and optimum temperatures usually enhance plant growth.

Liquid sprays more easily penetrate leaf surfaces as relative humidity increases. This increased penetration occurs because spray droplets do not evaporate as fast on the leaf surface. This allows more time for the herbicide to enter the leaf.

Light affects photosynthesis, plant growth, and flowering. Light is the energy source that drives photosynthesis. The rate of photosynthesis increases with increased light (up to about one-third of full sunlight). Plants growing in moderate shade tend to be taller and have larger leaves than the same species growing in full sunlight. Leaves in the sun usually are thicker, smaller, with have more cuticle than plants in shade. Some plants require high light intensity. Shade intolerant plants often establish first on disturbed sites because they grow well only in a lot of sunlight. On the other hand, shade tolerant plants become established later because they have the capacity to survive and grow in the shade of other plants.

Although plants come in numerous shapes and sizes, they can be grouped by a number of similarities for vegetation management purposes. All plants, including grasses, broadleaves, or woody plants, go through similar growth stages and have specific life cycles. In general, they respond similarly to environmental influences, although some plants may be more adapted to environmental extremes than others. Herbicides work best when plants are actively growing. Any conditions that make it difficult for plants to grow will reduce the effect of herbicides used to control them. Slow growth means the movement of water, nutrients, and herbicides are greatly reduced. Understanding the biology of weeds can help in planning effective management programs, regardless of whether you are trying to prevent, suppress or release them.

1. What type of plants is characterized by narrow upright leaves, parallel veins running the length of the leaf, one leaf at germination, and a fibrous root system?
    A. Trees
    B. Broadleaf forbs
    C. Grasses
    D. Ferns

2. What type of plants are characterized by leaves with net-like veins, two leaves at germination, a taproot or coarse root system, buds in each leaf axil, and not woody?
    A. Trees
    B. Grasses
    C. Broadleaf forbs
    D. Ferns

3. Seedling weeds are more easily controlled because:
    A. Wax and cuticle are less thick
    B. There are generally fewer and shorter hairs
    C. They are small in size.
    D. All of the above

4. Perennial weeds are difficult to control because they have:
    A. High seed viability
    B. Thick leaf cuticle
    C. Persistent root system
    D. All of the above

5. Plants which complete their life cycle in 1 year are called:
    A. Winter annuals
    B. Summer annuals
    C. Biennials
    D. Both A and B

6. Perennial weeds are those that live:
    A. One year
    B. Two years
    C. More than two years
    D. None of the above

7. Plants with a two-year life cycle are called:
    A. Annuals
    B. Biennials
    C. Perennials
    D. None of the above

8. Soil applied herbicides move upward in what plant tissue?
    A. Cambium
    B. Xylem
    C. Stomata
    D. Phloem

9. The manufacturing of sugar from water and carbon dioxide in the presence of green chlorophyll with sunlight as the energy source is termed:
    A. Assimilation
    B. Respiration
    C. Transpiration
    D. Photosynthesis

10. The correct sequence of plant growth stages is:
    A. Seedling, vegetative, mature, reproductive
    B. Seedling, reproductive, vegetative, mature
    C. Seedling, mature, vegetative, reproductive
    D. Seedling, vegetative, reproductive, mature