Plants that have erect, spreading, or prostrate habits of growth possess different 
herbicide-recipient characteristics and retention-capacity patterns. Broad, large, narrow, 
short, or linear leaf shapes greatly influence herbicide reception, distribution, and 
retention patterns. Plants with upright, horizontal, or pendulous leaves which form 
foliage canopies of various densities differ markedly in retention and uptake of herbi- 
cides, 
The nature of the leaf surface is critically important, Leaves with waxy, hairy or 
variously sculptured leaf surfaces differentially retain and absorb herbicides. Stomate 
distribution and nature of the cuticle probably determine the quantity of material that 
penetrates the outer layers of the leaf. The permeability barriers of the cell membranes 
may further decrease the amount of chemical absorbed into the individual cells, 
The form, shape, size, density, distribution, and other characteristics of roots in 
relation to soil-applied herbicides also partly determine the amount of herbicide that 
actually comes in contact with the plant. 
Collectively, the morphological characteristics of plants act as ‘‘obstacles’* through 
which the herbicide must pass before the _ selective control of plant growth 
is achieved, 
TRANSLOCATION OF HERBICIDES IN PLANTS 
A voluminous literature exists on the translocation of herbicides in plants. No 
attempt has been made to review the significant results of these studies, Certain prin- 
ciples on the movement of herbicides have been established (figure 3), 
Numerous factors influence the translocation of herbicides. Once the herbicide has 
been absorbed into the plant it may be immobilized in the treated tissue by adsorption to, 
or conjugation with, cell constituents. When some herbicides are absorbed the molecule 
is probably metabolized prior to translocation. Since the downward movement of foliar - 
applied herbicides occurs primarily in the phloem (living tissue) and the upward move- 
ment of soil-applied herbicides takes place mainly in the xylem (nonliving tissue), all 
factors which influence plant growth and the transport of inorganic and organic solutes, 
water, and mineral elements also affect the movement of herbicides in plants. 
Herbicides are differentially translocated and the efficiency with which different plant 
species translocate herbicides varies greatly. For example, the intercalary meristem of 
certain grasses restricts the translocation of foliar-applied herbicides, Certain trans - 
location barriers also exist in perennial herbaceous and woody plants. 
There is increasing evidence that lateral movement ofherbicides between the phloem 
and xylem occurs. The mechanisms, energy concepts, and interrelationships of the trans- 
location of herbicides in plants are not sufficiently understood for their most effective 
use. 
Our present knowledge of the translocation of herbicides, nevertheless, indicates that 
there are ‘‘translocation obstacles’’ which prevent herbicides from reaching their site(s) 
of action in the cell in sufficient quantity to control plant growth, These same obstacles 
also influence the differential selectivity of herbicides. 
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