WIREWORM POPULATIONS IN RELATION TO CROP PRODUCTION: III. POPULATION CHANGES AFTER SUMMER PLOUGHING

Six permanent pastures broken up in the summer of 1943 were sampled once a fortnight after ploughing in order to study changes in the wireworm populations.
The graphs of the running means of four consecutive counts showed the chief trends in population. On four of the fields, a slight increase in population was apparent up to December. Only in February and March was a definite decline apparent. On the other two fields, the counts declined by about 30 % during the autumn and winter.
A marked fall in population (from 30 to 70%) was evident on all the fields only after the first crop had been harvested. The new population at this time was characterized by an absence of larvae less than 4 mm. in length.
The results considered alone indicate that after summer ploughing the wireworm population remains unaffected until the following spring and summer during the cropping period. When the results are considered in relation to the grass 'controls', it appears that a net decline in population occurs during the autumn and winter and that the decline in spring and summer is not more marked than that which normally occurs at this time in grass fields. Unlike the grass fields, however, the reduction is permanent.     

WIREWORM POPULATIONS IN RELATION TO CROP PRODUCTION: IV. POPULATION CHANGES DURING A BARE FALLOW

Changes in wireworm populations during a bare fallow were studied on twenty-three fields in 1943 and 1944. In every case populations were much reduced, sometimes to less than 10% of the original level. Wireworms of all sizes were affected, but the larvae less than 5 mm. in length were practically eliminated from the populations.
Evidence is presented that the reduction achieved is greater in fields ploughed in February and March than in fields ploughed in May.
As the populations in the grass 'controls' decline during the same period the entire reduction cannot be attributed to the fallow. The reduction in the bare fallow, however, differs from that in the controls in that it is permanent and continues through the autumn when the counts on the 'controls' are rising.
The practical implications of the results are discussed.     

作物生产是一个种群过程

从分析自然选择和人工选择在目标和方向上的重大差异入手,指出自然选择的核心是生存竞争,其结果产生个体优势;人工选择的方向是群体优势,其目标是获得较高的经济产量。作物是典型的人工选择的产物,研究作物的生产过程就庆当以降低个体的竞争能力和提高繁殖分配为主,据此,对小麦植株高度,根系特征以及种群生态学在农业中的应用进行了讨论     

THE CHANGING NUMBERS OF APHIS FABAE SCOP., FLYING AT CROP LEVEL, IN RELATION TO CURRENT WEATHER AND TO THE POPULATION ON THE CROP

Aphis fabae Scop, flying over bean crops in the summer usually show a double peak of aerial density during the day; scarcely any flight occurs at night. The first peak is thought to be composed mainly of alatae moulted since the previous evening and the decline of the peak to their depletion as they fly away from the crop. The second peak is probably composed mainly of alatae moulted during the same day. Lack of flight at night is due partly to low temperature, partly to lack of alatae old enough to fly and probably also to low light intensity.
Contrary to expectation changes in aerial density from hour to hour are only very weakly correlated with weather factors, especially wind-speed. The total numbers in each of the two peaks do show a low but significant correlation with both wind-speed and temperature in one case, but even then only a relatively small amount of the variation in aerial numbers is associated with weather changes. Other factors, particularly rapid changes in the numbers of alatae on the crop caused by moulting, accumulation before flight and depletion by flight evidently obscure changes due simply to varying flight behaviour. Thus because of large populations on the crop, it is possible to have quite large numbers in the air even when the weather is relatively unfavourable for flight.
In addition to these observations, the relation between total numbers caught at different wind-speeds shows that most of the migration occurred in winds when the aphids could have had no control over the general direction of flight. The current view that most migration takes place only in calm weather can therefore no longer be held.     

A POPULATION MEMETICS APPROACH TO CULTURAL EVOLUTION IN CHAFFINCH SONG: DIFFERENTIATION AMONG POPULATIONS

We investigated cultural evolution in populations of common chaffinches (Fringilla coelebs) in the Atlantic islands (Azores, Madeira, and Canaries) and neighboring continental regions (Morocco and Iberia) by employing a population-memetic approach. To quantify differentiation, we used the concept of a song meme, defined as a single syllable or a series of linked syllables capable of being transmitted. The levels of cultural differentiation are higher among the Canaries populations than among the Azorean ones, even though the islands are on average closer to each other geographically. This is likely the result of reduced levels of migration, lower population sizes, and bottlenecks (possibly during the colonization of these populations) in the Canaries; all these factors produce a smaller effective population size and therefore accentuate the effects of differentiation by random drift. Significant levels of among-population differentiation in the Azores, in spite of substantial levels of migration, attest to the differentiating effects of high mutation rates of memes, which allow the accumulation of new mutants in different populations before migration can disperse them throughout the entire region.     

荔枝胚珠中多胺含量变化与胚胎发育的关系

荔枝胚胎发育与胚珠中３种多胺（ＰＡｓ）含量及其比例变化关系密切。试验结果表明：正常发育的胚珠中腐胺（Ｐｕｔ）、亚精胺（Ｓｐｄ）和精胺（Ｓｐｍ）的含量在胚胎发育的各个阶段均高于败育胚珠,并在花后７ｄ即达到最高值,其中Ｐｕｔ的含量最高,随后都呈下降趋势。但正常胚珠中Ｓｐｍ含量在花后２２至３１ｄ（球形胚至心形胚发育阶段）均有所回升,而败育胚珠无此现象。败育胚珠中的Ｓｐｄ和Ｓｐｍ在胚胎败育期的下降速度显著     

青藏高原草地植被覆盖变化及其与气候因子的关系

为揭示气候变化对青藏高原草地生态系统的影响及其生态适应机制,利用1982~1999年间的NOAA/AVHRR NDVI数据和对应的气候资料,研究了近20年来青藏高原草地植被覆盖变化及其与气候因子的关系。结果表明,18年来研究区生长季NDVI显著增加(p=0.015),其增加率和增加量分别为0.41% a^-1和0.001 0 a^-1。生长季提前和生长季生长加速是青藏高原草地植被生长季NDVI增加的主要原因。春季为NDVI增加率和增加量最大的季节,其增加率和增加量分别为0.92% a^-1和0.001 4 a^-1;夏季NDVI的增加对生长季NDVI增加的贡献相对较小,其增加率和增加量分别为0.37% a^-1和0.001 0 a^-1。3种草地(高寒草甸、高寒草原、温性草原)春季NDVI均显著增加(p<0.01;p=0.001; p=0.002); 高寒草甸夏季NDVI显著增加(p=0.027),而高寒草原和温性草原夏季NDVI呈增加趋势,但都不显著(p=0.106; p=0.087);3种草地秋季NDVI则没有明显的变化趋势(p=0.585; p=0.461; p=0.143)。3种草地春季NDVI的增加是由春季温度上升所致。高寒草地(高寒草甸和高寒草原)夏季NDVI的增加是夏季温度和春季降水共同作用的结果。温性草原夏季NDVI变化与气候因子并没有表现出显著的相关关系。高寒草地植被生长对气候变化的响应存在滞后效应。     

CHANGES IN PROTOPLASMIC CONSISTENCY AND THEIR RELATION TO CELL DIVISION

1. The development of the amphiaster is associated with the formation of two semisolid masses within the more fluid egg substance. 2. The elongation of the egg during cleavage is possibly produced as a consequence of the mutual pressure of these two growing semisolid masses. 3. The division of the egg into two blastomeres consists essentially in a growth, within the egg, of two masses of material at the expense of the surrounding cytoplasm. When all the cytoplasm of the egg is incorporated in these two masses cleavage occurs. 4. After a certain period of time the semisolid masses revert to a more fluid state. In the eggs studied this normally occurs after the cleavage furrow has completed the separation of the two blastomeres. The formation of the furrow, however, may be prevented in various ways, upon which the egg reverts to a single spherical semifluid mass containing two nuclei. 5. An egg mutilated during its semisolid state (amphiaster stage) may or may not revert to a more fluid state. If the more solid state is maintained, the cleavage furrow persists and proceeds till cleavage is completed. If the mutilation causes the egg to revert to the more fluid state the furrow becomes obliterated and a new cleavage plane is subsequently adopted. 6. The nuclei of eggs in the semifluid state are able to alter their positions. In semifluid mutilated eggs the nuclei tend to move to positions which may assure symmetry in aster formation and cleavage.     

Cherelle Wilt of Cacao: II. WILT IN RELATION TO YIELD

Experiments were carried out to study whether cherelle wiltis responsible for loss of yield in cacao. It was found thatincreasing or decreasing flower setting had no effect on yieldas the crop was unrelated to the number of cherelles formed.Preventing setting till after the April ‘flush’had no effect on yield. Variations in wilt between and withinselections were studied, and it was concluded that yield isdependent on the nutrient status of the tree and independentof fluctuations in cherelle wilt.