Biological and taxonomical investigations of some oak species
Hermaphroditic flowers blooming unseasonably on an oak tree (»the green oak«) of uncertain hybrid origin prompted a detailed investigation into the biology of the flowering, and the morphology of various traits of the tree, which is growing in an urban area near Zadar, northern Dalmatia, Croatia. Hermaphroditic flowers were also detected on one holm oak tree from Weihenstephan, Germany. Since the first oak tree was described as a hybrid between Q. cerris f. austriaca Q. ilex, scanning electron microscope investigations of pollen and hairs on the leaves of three species were made, on: Q. ilex L., Q. cerris L. and the »green oak«. Oak pollen is of elliptical shape and tricolpate. The ornamentation of the pollen grains of Q. cerris and the »green oak« were similar concerning the ornamentation of the exine – warty and rough and with small rounded protrusions – as well as the size of the pollen grain. Q. ilex pollen was much smaller and the surface lacked the small irregularly shaped protrusions typical of the other oaks. Within samples of holm oak pollen, significant difference in the surface structures was observed. Concerning the different types and number of hairs on the leaf surfaces, Q. ilex revealed a much greater number of stellate hairs on the upper leaf side as compared to Q. cerris and the »green oak«. Simple uniseriate hairs and simple unicellular hairs were missing on the leaves of Q. ilex, but were present on both the other oaks. To sum up the results, the »green oak« and Q. cerris reveal some similarities whereas Q. ilex does not.
Anatomy of Quercus × viridis Trin., an oak of unknown origin in Croatia.
Near Zadar, Croatia, grows an oak which is different from other local oak species in the neighborhood: Q. x viridis Trin., called green oak. To clarify the origin of this tree we investigated the anatomy of wood and leaves (in particular, the hairs and epidermis cells were examined). The investigations were done with light and scaning electron microscopic (high vacuum and ESEM) techniques combined with digital image analysis. Wood anatomy results from Q. cerris, Q. ilex and Q. pubescens were compared with data from the literature. Q. cerris, Q. ilex and Q. pubescens are supposed to be the ancestors of Q. x viridis. Investigations of the leaves were done with Q. x viridis, Q. cerris and Q. ilex. Number of measurements and histochemical reactions were done with the epidermis, hair, and root-meristem cells. EDX was used to determine the elements in the hairs, and the DNA-content of the nuclei was measured using Feulgen-reaction in combination with a densitometric method. 
The wood of Q. x viridis, Q. cerris and Q. pubescens is ring-porous, and of Q. ilex is difuse-porous. The anatomy of Q. x viridis and Q. cerris is very similar, triangulary ray cells were seen in preparations of these trees only. Q. x viridis has five different types of hairs, the other investigated oaks only four. Q. x viridis and Q. cerris have a similar number of hairs on the adaxial leaf surfaces. The number of hairs on the abaxial surface is different, Q. x viridis has less hairs. The size of hairs of Q. x viridis and Q. cerris is similar. We found lipoids and cuticle in all hairs, only the cell wals of the hairs of Q. x viridis are slightly woody. EDX analysis shows no flint or lime. DNA measurements have shown contents of 1.2 pg for 2 C nuclei of Q. x viridis, and 1.4 pg (root-meristem) for Q. ilex. The majority of the nuclei of hair cells have a DNA content of 2 C (all investigated oaks), some of them 4 C and 8 C (Q. cerris). The nuclei of epidermis cells have DNA content up to 16 C. Result of this investigation and the origin of the green oak is discussed.
Global attractivity in time-delayed predator-prey systems
Abstract For a predator-prey model with time-delay due to gestation, criteria are obtained for persistence and global attractivity. The global attractivity criteria apply only to models with a decreasing prey isocline.