The employment of selective herbicidal sprays to kill or inhibit one or more undesirable plant species growing in close association with desirable species has become a common practice in producing certain crop and pasture species. Many crop and weed plants which are equally inhibited if exposed to certain herbicides during the germination period may show marked differential response when the tops of the same plants are sprayed with the materials. For example, the common small grains and corn are readily inhibited by the herbicide 2,4–dichlorophenoxyacetic acid (2,4–D) if exposed to solutions of the herbicide at the germination stage. Notwithstanding, 2,4–D is widely used in spraying these crop plants at certain stages to control weeds without causing significant damage to the crops. The lack of serious damage to these species from applications of 2,4–D to their tops may be attributed to several factors. Firstly, the spray may be repelled and shed harmlessly to the soil by the crop species and retained by the weedy ones. Secondly, the spray may be retained about equally well by the foliage of all species, but absorption and translocation of the herbicide to susceptible tissues, such as the stem and root apical meristems, may be less efficient in the non-susceptible species than in the susceptible ones. Possible differential accumulation of herbicides at the site of toxic action by susceptible and tolerant plants also may be important. Thirdly, the differential response of plant species to certain herbicidal sprays may be attributable to an inherent physiological capacity of the non-susceptible plant to tolerate the herbicide even in intimate association with the cellular components. Fourthly, some herbicidal materials appear to be readily taken up by the normal absorbing organ, the roots, of plants, but are not absorbed efficiently by the aerial plant parts because of certain physio-chemical relationships and internal morphological barriers. Fifthly, the position of the stem growing point may be a factor influencing the differential response of plants to herbicidal sprays. The apical bud of the stem in most broadleaf plants is usually wet by foliage sprays, whereas the stem bud of grasses is protected by several enveloping leaves. Sixthly, some species may inactivate the herbicide more rapidly than others and hence are not highly responsive.