Plant groups annuals

Paleobotanists commonly collect and identify microscopic spores, pollen, and bits of larger tissues. They also may identify larger, macroscopic plant remains such as leaves and even fossil tree trunks. Often, only the major plant group to which these plant parts belong, such as order or family, can be identified. In the case of more recent plant fossils that represent species that are still extant (not extinct), the remains may even be identifiable down to genus or species. Sometimes, the age of samples is known quite accurately. Paleobotanical studies of some recent lake sediments have shown that sediment layers sometimes develop as annual accumulations. The total number of layers can be subtracted from the current year to determine an age for the sequence or any layer within. 

There are many ways of classifying plant groups but, from an agricultural and horticultural point of view, a useful way is to divide them into annuals, biennials and perennials according to their total length of life. 

The chlorophylls are a group of naturally occurring pigments produced in all photosynthetic plants including algae and some bacteria. Hendry24 estimated annual production at about 1,100,000,000 tons with about 75% being produced in aquatic, primarily marine, environments. Obviously as a source of raw material for food colorants, chlorophylls present no problem with supply. 

Unfortunately, pure xylan is an expensive carbon source for commercial-scale xylanase production. Therefore, several groups have tried to develop xylanase production on cheaper xylan-rich materials. The best candidates for the purpose appear to be water-soluble hemicellulose from steam-treated wood (63,69) and residues of annual plants like wheat bran (70). 

Hardwood xylans and xylans of annual plants may contain up to 7% O-bound acetyl groups. Seven out of ten xylose residues in native hardwood xylan are acetylated on C-2 and/or C-3 (10). Because of the possible migration of O-acetyl groups between 2- and 3-positions during and after isolation of hemicellulose components, it is difficult to determine their original distribution in nature (11). The ratios reported for 2-, 3-, and 2,3-positions of acetyl groups in birch xylan have been 2 4 1 (3) and 2 2 1 (10) and in bracatinga xylan 3 3 1 (12). 

The most economically important group of natural plant insecticides are the pyrethrins, a group of six closely related esters extracted from pyrethrum (Chrysanthemum clnerariaefollum) flower heads (Figure 1). Pyrethrum has been used as an insecticide since at least the early 1800 s in Persia and Yugoslavia. By 1828 pyrethrum was being processed for commercial Insect control, and by 1939 imports of pyrethrum into the United States reached a peak of 13.5 million pounds. Use of the natural product declined in the early 1950 s because of the advent of synthetic pyrethroid analogs (for example, allethrins), which were both more stable and more effective in the field. The present worldwide demand for pyrethrum flowers remains in excess of 25,000 tons annually and is satisfied by the estimated 150 million flowers still hand-harvested daily, predominantly in natural stands and cultivated fields in Kenya, Tanzania, and Ecuador (9 ). 

The triazine herbicides currently used are mostly 4,6-alkylarmno-v-triazine compounds with either a 2-chloro, 2-methylthio, or 2-methoxy substituent (Table 23.1). The /V-alkyl groups may be methyl, ethyl, 1-methylethyl (isopropyl), 1,1-dimethylethyl (tertiary-butyl), 1,2-dimethylpropyl, or 2-methylpropanenitrile. Absorbed by roots or leaves of plants, these herbicides are applied either preemergence or postemergence to control annual broadleaf weeds and annual grasses in a wide variety of crops. The triazine herbicides listed in Table 23.1 have the same mechanism of action in plants, as all are photosynthetic electron transport inhibitors. 

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