Use of centrifugal force in the study of xylem cavitation

Two methods were evaluated for using centrifugal force to measurethe occurrence of cavitation as a function of negative pressuresin xylem. The general protocol was to measure the hydraulicconductivity of xylem segments (stem or root pieces) beforeand after centring them on a centrifuge rotor and spinning themabout their long axis to generate negative xylem pressure. Thepercentage decrease in conductivity from the initial to finalmeasurement was used to quantify the embolism resulting fromcavitation during spinning. In one approach, segments were spunwith their ends exposed to air. This method could only be usedwhen xylem conduits were much shorter than the segment. Resultsfrom an angiosperm (Betula occidentalis) and a gymnosperm (AblesIasiocarpa) corresponded to previous observations of embolismcaused by air dehydration where negative pressure was measuredwith the pressure chamber. Results also agreed with embolismcaused by injection of air into the xylem, in support of theair-seeding hypothesis for cavitation. In a second approach,segments were spun in a rotor designed to keep the segment endsimmersed in water during spinning. This gave the same resultsas for non-immersed segments. Immersing the segment ends allowedmeasurements on any material, regardless of conduit length,as demonstrated for roots of B. occidentalis. The chief advantageof the centrifugal force method is the rapidity and precisionwith which any desired xylem pressure can be imposed. Key words: Cavitation, embolism, drought stress, water relations, water transport     

Topography and nerve supply of the cucullaris (trapezius) of skates

Dissections of Sudan black B stained specimens reveal that, of a complex of medial, intermediate, and lateral muscles of skates, presumed homologous to the cucullaris of sharks, only the lateral muscle is innervated by a branch or branches of the vagus and is inserted, in part, to the fused pharyngobranchials of the caudal visceral arches. The medial and intermediate muscles are supplied by separate branches of rostral spinal nerves and do not attach to the branchial skeleton. The lateral muscle therefore is the most likely homologue of the cucullaris (trapezius) of sharks and perhaps other fishes and tetrapods. The medial and intermediate muscles appear to be part of the axial musculature.