Sex expression
A tree consists of numerous branches whose shoots may or may not carry an inflorescence which consist of an aggregation of flowers. To understand the sex expression of a tree population one has to examine the sex expression at the level of the flower, the inflorescence, the individual and finally the population.
THE FLOWER
With the exception of some male flowers (
I), which
instead of a gynoecium have only a tuft of white hairs,
all flowers are morphologically hermaphrodite, but are
functionally unisexual. II flowers have a rudimentary
gynoecium which never develops, and flowers have stamens
which never grow and their pollen never released (Fig. 1).
Weiser (1973) recorded a few hermaphrodite flowers,
whereas in Ireland I have never observed functionally
hermaphrodite flowers, but I did record a few flowers
where the development of the gynoecium stopped while some
of the anthers reached anthesis. Asexual flowers occur on
some inflorescences at the end of the flowering sequence.
In the following, all functionally male or female flowers
will be described as male or female flowers.
Figure 1. Flower types in sycamore.
A. I
flowers: morphologically and functionally male,
B. 
I flowers: morphologically hermaphrodite but functionally female,
C.
II flowers: morphologically hermaphrodite but functionally male (drawings from de Jong 1976.)
Each anther produces on average 2935 pollen grains (Pohl 1937) which are 60±4.4µ long and 28±2.5µ wide (Semm 1965). 32.8% (range 22.7% to 59.5%) of the male flower pollen germinates and none in female flowers (Svobodová 1977).
Female flowers produce during the whole flowering season more nectar (5.92mg) than male flowers (4.87mg) and a higher proportion of sugars (respectively 43.2% and 39.0%) (Haragsim 1977). The nectar is composed of 88.9% sucrose, 3.7% fructose and 7.4% glucose (Ivanov & Vachev 1984) and in spring the sycamore is a very important source of nectar and pollen for bees (see Maurizio 1953, Pritsch 1961, Wille et al. 1985).
SEX EXPRESSION OF INFLORESCENCES AND INDIVIDUALS
Some sycamore inflorescences bear male flowers only, but generally both sexes are represented. Usually inflorescences start flowering with a sequence of male or female flowers and then switch to the other sex. Up to five changes in the sex expression on any one inflorescence has been observed by de Jong (1976). De Jong (1976) using the classification of Wittrock (1886), as modified by Correns (1928), observed 11 modes of sex expression of inflorescences but I have only recorded five Modes in Ireland (Fig. 2).
Figure 2. Modes of sex expression observed on sycamore inflorescences in the north of Ireland (modes of sex expression after Wittrock 1886).
The mode of sex expression of individuals is similar, but is a mixture of the above modes of sex expression. In most cases, all inflorescences of a tree always start flowering with a male or female sequence and switch to the other sex one or more times, but the switch is not normally synchronised on the whole canopy. An individual may therefore be classified as protogynous (female starters) or protandrous (male starters). A few individuals flower solely with male inflorescences (classified as protandrous as well) whereas on a few individuals, some inflorescences may start with a small number of flowers of the opposite sex.
Some year to year variation in sex expression may be observed. Some individuals, which normally bear protogynous inflorescences only, may occasionally be observed to have protandrous inflorescences. Such inflorescences always start with a very short male sequence and in Ireland only represented about 3% (N = 4) of the protogynous trees observed over three years, whereas de Jong (1976) found that 15.8% of the protogynous trees showed a shift to male flowering start over four years. Protandrous individuals are very variable, as the proportion of the different types of protandrous inflorescences varies greatly from year to year.
Several other more minor and rarer variations can also be observed. Such variations are to be expected, because 1/ morphologically, sycamore is a very variable tree (see Section 3) and 2/ it has been shown that sex expression in plants is not stable and may be strongly affected by environmental conditions (Freeman et al. 1980).
FLOWERING PHENOLOGY
The duration of anthesis appears to vary depending on weather conditions and on the position of the flower on the inflorescence and may last a few days. During cold periods I noted that few flowers reached anthesis and the converse was true during hot sunny days. Because there are many more male flowers on an inflorescence, the duration of the male flowering sequence is always much longer (about ten days) than that of the female sequence (up to six days). On individual trees variation between the onset of anthesis between different parts of the crown may be large, allowing for potential self-pollination, and this is also possible on some inflorescences, particularly protandrous ones, where there is some overlap between sex sequences. In coastal areas of Northern Ireland peak flowering time occurs during the last week in May and the first week in June, but in any one population there may be a difference of up to one month between the first and the last tree reaching anthesis. Annual variation in flowering time is about ten days and is much less than surrounding grassland vegetation which is approximately 3-4 weeks. In any one population most protandrous trees always reach anthesis before protogynous individuals.