A Population Study of Spergula arvensis: I. Two Clines and their Significance

Two seed-coat forms (the presence or absence of papillae) andtwo hairiness forms (medium or densely hairy) of Spergula arvensisare differently distributed through the British Isles, theirrelative frequencies being correlated with latitude and altitude.The proportions of the non-papillate and densely hairy formsincrease with increasing distance from Kent to the north-north-westand also with increasing altitude. A possible explanation is that the diverse forms indicate selectivedifferences. Experiments have been carried out to test thishypothesis and the evidence supports it. The non-papillate formproduces a lower proportion of fertile capsules than the papillateform when grown at a high temperature and low humidity. Alsothe non-papillate seeds germinate more readily than the papillateseeds at low temperatures, whereas at higher temperatures thereverse is true. It is suggested that hybrid vigour, fluctuating selection intensities,and migration may all play a part in maintaining the observeddistribution of the various forms.     

Microarchitecture of Body Wall of Extant Cyclostome Ectoprocts

Scanning electron microscope and light microscope studies ofthe body wall in cyclostome stenolaemate ectoprocts show thatthis wall has a distinctive microarchitecture. The microstructuresconsist of a linear series of longitudinal canals in the centralregion. Adjoining regions on either side have laminae of overlappingcalcareous tablets. In some cyclostomes at certain stages ofgrowth, laminae may show a less orderly meshwork pattern nearthe canals. The laminate layers of calcareous tablets are enclosedin an organic matrix and are penetrated by tubuli which passfrom the inner part of the body wall to the canals and alsolink the canals. The canals with organic and mineralized matricesopen into pores in the body wall. This elaborate meshwork andcanal and pore system provide the framework for growth and resorptionof the body wall and facilitate transport of cellular materialsfrom one zooid to another in a colony. Calcification at thedistal part of a colony seems to proceed in a series of stagesaround sites of calcification. The pattern of the body wallof the cyclostome ectoproct does not parallel a molluscan patternof skeletal growth as previously proposed. The hypothesis ofa hypostegal coelom in some cyclostome ectoprocts is rejected.