Spines may be considered as one of the most specific morphological structures in the Cactaceae family plants. Diversity of spines in plants of this family is really mind-boggling (see photo 1) being conditioned by genetic as well as environmental factors. Peculiarities of spines structure take important place in taxonomical diagnostics of cacti (to prove it any taxonomical description of these plants will do). Plenty of serious publications are dedicated to macro- and micro-morphology analysis, ontogenetic and phylogenetic development of cacti spines (see e.g. Buxbaum, 1957; Leinfellner, 1937; Schill et al., 1973; Rauh, 1979). However there is considerably less knowledge about biological importance of spines, and discussions of the point often tend to be speculative. This is true in particular for the question of possible contribution of spines to water exchange in cacti. Existing specific proofs are based on peculiarities of micro-relief of spines in some species and on certain experiments with radioactive marks (Schill, Barthlott, 1973; Barthlott, Capesius, 1974; Rauh, 1979).

Photo 1. Types of cacti spines ("Cacti Journal", #7/1988, Sumy)

1. Bristled (Cephalocereus senilis)
2. Acicular (Opuntia santa-rita)
3. Subulate (Austrocylindropuntia subulata)
4. Conical (Cereus xanthocarpus)
5. Tortuous (Ferocactus acanthodes)
6. Plumose (Mammillaria plumosa)
7. Sheathed (Opuntia tunicata)
8. Pectinate (Solisia pectinata)
9. Hooked (Ferocactus wislizenii)
10. Appressed (Gymnocalycium denudatum)
11. Glochids (Opuntia microdasys)
12. Annulate (Homalocephala texensis)
13. Paper-like (Tephrocactus articulatus v. papyracanthus)
14. Deflexed
15. Porrect (Opuntia elata)
16. Down-curved

That's why we thought it would be interesting to analyze wettability of cacti spines. One of the authors of this article (T.N.Sidorova) has been participating in investigations dealing with wettability of hairs of some sea mammals and feathering of certain aquatic birds. It has been shown that mammals' hair and birds' feathers come in two types: hydrophobic and hydrophilic (Romanenko, Sokolov, 1987). Structural diversion of cacti spines is quite comparable with hairs as well as feathers (many spines by the way are strikingly similar in exterior to them). Besides there is an exceptionally wide range of hydrological conditions in areas where cacti grow (Rauh, 1979) and we may expect to come across spines with various degree of hydrophobic and hydrophilic properties. Evidently both of these properties may be useful for cacti since their assumed relations with the atmospheric moisture are fairly varied – rain and condensate absorption , transportation of water from spines to the stem or soil, protection of stem from wetting, change of micro-thermal conditions around the stem depending on humidity.

MATERIALS AND METHODS

For our aims there have been used spines of cultivated plants of 14 species from the personal collection of one of the authors (D.V.Semenov). While selecting plants we tried to represent in our investigation taxonomic variety of cacti, basic types of spines and species from different areas of growth. Furthermore we used for comparison not only taxonomically distant plants but also closely related ones (e.g. two plants from subspecies Eriocactus). Only mature, full-grown, normally developed and clean spines were selected. The spines had been separated at the base or removed together with the areole. For every species there have been used several spines from one or two specimens. In case a certain species had several types of spines (e.g. Mammillaria guelzowiana) we analyzed them all.

The methodology of studying wettability of external integuments in plants and animals has been worked out fairly well. Hydrophobic properties of any object surface are defined by the so called "wettability angle", that is, the angle between substrate surface and the droplet on it. When wettability is complete the angle is equal to zero (the drop spreads about the surface). Absolute non-wettability is defined by the 180( angle (a water drop moves on the surface as if it were a ball).

In our work we used a modified version of this methodology (Romanenko, Sokolov, 1987). The object was placed under the binocular with a photo extension piece and sprayed with distilled water. Then a series of shoots were made. Macro-survey allows to evaluate an extent of micro-drops spreading on various parts of spines and even micro-hairs of areoles. Using a protractor, contact angles were measured on the photos. There have been done 1532 measurements all in all. The authors realize that common methods do not allow to measure genuine contact angle. In fact we measured an angle of liquid's surface inclination relative to the plane with projected solid surface profile – a macro-contact angle, to be exact. But we use an established term "contact angle".

Average values of these angles are determined by standard statistical procedures and give unbiased enough picture of wettability in investigated objects. The photos made at different time intervals after the spraying allow to evaluate wettability versus water contact duration. Macro-photos helped also to evaluate peculiarities of spines surface microstructure.

DISCUSSION

The results show a rather clear-cut relationship between wettability as a physical-chemical index and the definite types of spines, and, perhaps, a plant's species. This is shown by good repeatability of experiments results (for the same spines; for different spines of one type from the same plant; for the spines of one type from different plants of the same species; for the spines of one type from the plants of closely-related species). Moreover the statistical error is small in practically all series of measurements. Also there was revealed quite a broad range of wettability index – an average contact angle for different objects lies in the range from 16 up to 105 degrees. It should be mentioned that for certain types of spines the range is significantly narrower, thus the spines with different morphology also differ in their wettability.

It is known that wettability indices describe mainly physical-chemical peculiarities of cacti spines surface. General state of the surface depends on many factors and in particular from moisture content (that's why repeated spraying increases hydrophilic properties of surface).

It is also known that possibility and speed of liquid spreading on the genuine solid body surface depend on its homogeneity, purity and relief. Also surfaces of some polymers are able to change their conformation and orientation of polar groups under the influence of wetting liquid. Thus the initial conditions for wetting and spreading of liquid are changed and establishment of equilibrium contact angle is slowed. Micro-relief unevenness is another factor affecting contact angle. But it reveals itself only when this unevenness reaches certain degree (Summ, Goryunov, 1976; Marmur, 1983).

The wetting proper may be physical (reversible) with molecular forces participating in it and provisionally chemical (non-reversible) when forces of chemical interaction prevail. In the examples discussed above some of the objects obviously interact with water according to the first type (Ferocactus, Mammillaria), the wettability of the others is closer to the second type (Leuchtenbergia) (it's possible that in the latter case we observe the effect of a porous body wetting). The difference between observed contact angles after spraying dry and pre-wetted spines is a representation of hysteresis phenomenon.

Interpretation of the results we received is impeded by the fact that many properties of spines remain unknown, in particular their physical-chemical composition, presence of alien substances on their surface. It is however clear that wettability of spines is defined by their morphology and other peculiarities of their structure.

Importance of condensed moisture as a water source for plants from arid areas is often discussed in cacti literature. It seems that well wettable spines may "collect" moisture and draw it to the stem epithelium. On the contrary dew on the poorly wettable spines beads and then either evaporates or drops to the ground. It is clear that the process of water transportation from spines depends on their peculiarities and general architecture of a cactus "spiny cover". Indeed, hydrophilic paper-like spines of Tephrocactus and Leuchtenbergia are quite able to serve as conductors of moisture condensing on them to the interior of the plant through areole. All other types of spines are not good for that because they are not wetted well enough. Even when such a spine is set vertically a drop flowing down is detained by the hydrophobic bristles of the areole. It is interesting to note once again that there exists rather a curious phenomenon of differentiated wettability of certain parts of spines found in various species and spines with different morphological types. It is evidently bound up with the structural heterogeneity of spines as well as the process of their wear, e.g. "worn out" tips of hair-like spines of a white-haired Opuntia are considerably more hydrophilic than the basal parts of these very hairs. On the contrary, thick needle-like spines of a Ferocactus and hooked spines of Krainzia are more hydrophobic at the tips than at the base.

CONCLUSIONS

Wettability analysis of morphologically different spines of 14 cacti species has shown that this property is clearly bound up with certain types of spines. Paper-like spines are the most hydrophilic, needle-like and hooked spines are a bit more hydrophobic, and the hair-like spines and areole's hairs are hydrophobic to a larger extent. Feather-like spines proved to be the most hydrophobic ones.

Spines of certain cacti species are marked by the local differentiation in wettability characteristics. Apparently wettability of spines as an independent attribute is of no essential biological importance for the majority of cacti species. Nevertheless it should be taken into account for the species with paper-like spines when assessing water exchange in cacti.

REFERENCES

1. Kostina G.N., Sokolov V.E., Romanenko E.V., Sidorova T.N., Tarchevskaya V.A., Chernova O.F. Hydrophobic properties of penguin feather structures (Аves, Sphenisciformes)// Zool. journ., 1996, vol. 75, issue 2, p. 237-248.
2. Romanenko E.V., Sokolov V.E. Wettability of hairy cover of the northern fur-seal. // Acad. of Sc. of the USSR, 1987. Vol. 297, No 4, p. 990-995.
3. Schill R., Barthlott W., Ehler N.. Mikromorphologie der Kakteen-Dornen. // Trop. und Subtrop. Pflanzenwelt 1973, H-6, S. 1-32.