Weeds are often spatially aggregated in maize fields, and the level of aggregation varies across and within fields. Several annual weed species are present in maize fields before postemergence herbicide application, and herbicides applied will control several species at a time. The goal of this study was to assess the spatial distribution of multispecies weed infestation in maize fields. Ground-based imagery was used to map weed infestations in rain-fed maize fields. Image segmentation was used to extract weed cover information from geocoded images, and an expert-based threshold of 0.102% weed cover was used to generate maps of weed presence/absence. From 19 site-years, 13 (68%) demonstrated a random spatial distribution, whereas six site-years demonstrated an aggregated spatial pattern of either monocotyledons, dicotyledons, or both groups. The results of this study indicated that monocotyledonous and dicotyledonous weed groups were not spatially segregated, but discriminating these weed groups slightly increased the chances of detecting an aggregated pattern. It was concluded that weeds were not always spatially aggregated in maize fields. These findings emphasize the need for techniques allowing the assessment of weed aggregation prior to conducting site-specific weed management.

Companion cells in the secondary phloem of 125 Indian dicotyledonous species belonging to 43 families were examined by light microscopy. Four types of companion cell were identified: E-, S- and L-types which were equal in length, shorter, or longer, respectively, than the associated sieve tube element; and R-type, in which two or more companion cells in a vertical row were associated with the sieve tube element. The commonest was the E-type and the rarest was the L-type. E-type companion cells were most frequently found associated with short sieve tube elements (50–250 μm) which had a high frequency of simple sieve plates, considered phylogenetically advanced. R-type companion cells were most frequently associated with long sieve tube elements (>400 μm) with a high frequency of compound sieve plates, considered phylogenetically the least advanced. A strong positive correlation was found between the average number of companion cells associated with a sieve tube element and the lengths of the sieve tube elements. There was also a strong negative correlation between the average number of companion cells associated with a sieve tube element and companion cell lengths.