A comparative study on nutrient cycling in wet heathland ecosystems : I. Litter production and nutrient losses from the plant.

The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g-1 biomass year-1). It is derived that in a plant community with two competing perennial plant populations, species1 will become dominant if the relative competition coefficient k 12 (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L 1/L 2), whereas species 2 will become dominant, if k 12 is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g-1 yr-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g-1 yr-1). The relative competition coefficient of Molinia with respect to Erica (k me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k me is below the critical ratio of the relative nutrient requirements of the two species (L m /L e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.

Skull morphogenesis and growth: hemodynamic influence.

In young dogs movements of cranial bones at the sutura sagittalis and sutura coronalis have been shown. They correlate with arterial blood flow. The connective fiber arrangement in the sutura, which has been found earlier, can be explained as being related to these movements.

Sutural growth.

As the skull develops, the tissue of the coronal and sagittal sutures (serrated sutures) assumes a specific structure which in part is practically identical to that of a gomphosis joint and may therefore be regarded as a 'multigomphosis'. The nature of this structure justifies the assumption that it has to resist mechanical forces exerted on the suture. The results of transplantation experiments with portions of sutures suggest that the sutural structures are determined hereditarily, but that environmental factors are required for these qualities to manifest themselves.

Morphological changes in the coronal suture after replantation.

The coronal suture with the adnexa was excised in 1-week-old rats and replanted onto the exposed dura mater of the same rat. After replantation, the original internal part of the suture (= pars interna) was located on the external surface of the cranial vault, while the original external part (= pars externa) was located on the dura mater. The animals were sacrificed 1, 2, 3, 4, 5, 6 and 7 weeks after operation. The microscopic observations, done 3, 4, 5, 6 and 7 weeks after replantations, reveal that gradually such alterations occur in the replanted suture that the pars interna transforms into a pars externa and vice versa. These modifications are discussed in the light of the general articulations of the cranial bones. In the observed alterations of the replanted coronal suture we see a confirmation of the hinge hypothesis proposed by Oudhof.

A study on the occurrence of alkaline phosphatase in the sutura interfrontalis of Wistar rats.

The aim of the present study was to determine the presence of alkaline phosphatase during various stages in development and closure of the sutura interfrontalis. The histological sections reveal that this enzyme could primarily be demonstrated in the dura mater of this suture. In further developmental stages, alkaline phosphatase could be observed within the intermediate zone as well as the pericranium. These findings are brought in relation with the occurrence of synostosis which can be induced under experimental conditions.

The presence of alkaline phosphatase in the coronal suture of rat.

The purpose of the present study was to determine the presence of alkaline phosphatase within the coronal suture, the dura mater and the pericranium. The histologic sections reveal that no alkaline phosphatase activity could be observed within the coronal suture as well as the dura mater, while the pericranium did contain this enzyme. These findings are brought in relation with the occurrence of synostoses, which can be induced under experimental conditions.