Weeds are often spatially aggregated in maize fields, and the level ofaggregation varies across and within fields. Several annual weed species arepresent in maize fields before postemergence herbicide application, andherbicides applied will control several species at a time. The goal of thisstudy was to assess the spatial distribution of multispecies weedinfestation in maize fields. Ground-based imagery was used to map weedinfestations in rain-fed maize fields. Image segmentation was used toextract weed cover information from geocoded images, and an expert-basedthreshold of 0.102% weed cover was used to generate maps of weedpresence/absence. From 19 site-years, 13 (68%) demonstrated a random spatialdistribution, whereas six site-years demonstrated an aggregated spatialpattern of either monocotyledons, dicotyledons, or both groups. The resultsof this study indicated that monocotyledonous and dicotyledonous weed groupswere not spatially segregated, but discriminating these weed groups slightlyincreased the chances of detecting an aggregated pattern. It was concludedthat weeds were not always spatially aggregated in maize fields. Thesefindings emphasize the need for techniques allowing the assessment of weedaggregation 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.