| Popis: |
palatability. Comparison of crop plants with uncultivated species provides a marked contrast with respect to palatability. Many wild plants are known to be partially or wholly inedible to the majority of herbivores; chemical and physical features protective against grazing and browsing are various and effective. The appearance of such features as characteristics of genera (e.g. Ulex), tribes (e.g. Compositae-Cynareae) or possibly even families (e.g. Ranunculaceae) is perhaps evidence of a long history of selection against palatability. Despite the efficacy and immediate selective advantage of mechanisms protecting plants from consumption it is interesting to find that contemporary natural and semi-natural vegetation invariably includes plants which are palatable to many herbivores. Certain of these plants, e.g. Poapratensis,* Onobrychis viciifolia and Smyrnium olusatrum, have been subjected to periods of cultivation. Study of palatable but uncultivated plants may enable us to define the circumstances in which selection for palatability occurs in natural vegetation. There are additional problems associated with palatability which are worthy of attention. It would be of value to distinguish between selection pressures conducive to the evolution of repulsive plants and those favouring palatable species with growth forms adapted to withstand regular defoliation, e.g. many grasses. Herbivorous animals differ considerably in their effect on vegetation. In particular, differences occur between vertebrates and invertebrates. To larger mammals, only the top of the plant may be accessible. Gasteropods, on the other hand, tend to concentrate on the lower foliage. Other more subtle differences in the damage caused by herbivores arise from differences in features such as the size of the animal, the form and effectiveness of the mouth parts, the annual and diurnal pattern of feeding and dependence on various sensory organs for food-finding and discrimination. In view of these differences it would appear worth-while to classify the 'defence mechanisms' of plants in terms of the herbivores against which they are most effective. This may permit a calculation of palatability in any particular circumstance as the product of an interaction of plant and animal attributes. This paper is a contribution to the areas of study which have been outlined, and concerns an attempt to measure the palatability of leaves of a range of flowering plants to the snail Cepaea nemoralis. The plants examined include species of known palatability to |
