Temporal changes in community organization were examined in a 300+ year chronosequence of understorey vegetation data from lodgepole pine forests recovering from fire in central British Columbia. Changes between six age-classes of forest were quantified as shifts in the orientation of equal frequency ellipses depicting the main correlation structure of the vegetation in multivariate space. Different developmental trajectories were obtained for sites differing in soil moisture status. Mesic sites displayed sharp changes in community organization within the first 100 years following fire but only gradual changes thereafter. In contrast, xeric sites exhibited sharp organizational changes at the beginning and again toward the end of the chronosequence. The unanticipated behaviour of dry sites is interpreted as reflecting a lower degree of integration of such communities resulting from their particular species composition and susceptibility to biotic disturbance. Analyses of separate life-form strata indicated continuing organizational changes in shrubs, forbs–grasses, and lichens, but relative stability in bryophytes after 100 years. The movement through time of mesic sites towards increasing persistence is predicted from an interpretation of ecological succession as a process of self-organization, directed by principles of nonequilibrium thermodynamics. Keywords: postfire succession, Pinus contorta forest understorey, plant community organization, equal frequency ellipses, principal components analysis, self-organization, nonequilibrium thermodynamics.
In Stipa elmeri Piper & Brodie ex Scribn., the pollen tube enters at the filiform apparatus of the degenerated synergid. The degenerated synergid has electron-dense cytoplasm in which organelles are not discernible. All other cells of the mature megagametophyte have nuclei, endoplasmic reticulum, plastids, mitochondria, dictyosomes, and vacuoles. Starch is found in the persistent synergid (in minute quantities), egg, and central cell. Lipids occur in the persistent synergid, central cell, and antipodals. The filiform apparatuses of the two synergids are hypothesized to perform different functions. In the degenerated synergid, the filiform apparatus serves to increase the surface area of the plasma membrane and thereby to offer a large area for pollen-tube-growth-directing compounds to diffuse out of the synergid. In the persistent synergid, the filiform apparatus is part of a suite of features which indicate that the persistent synergid is involved in the transference of materials into the megagametophyte. Another possible function of the persistent synergid is to aid in establishing the polarity of the egg. The pollen grain and tube have distinctive polysaccharide spheres that serve to delimit the pollen tube cytoplasm after discharge into the degenerated synergid. Associated with the degenerated synergid are bodies of dense materials as seen under electron microscopy, and bodies of RNA and protein as determined histochemically. These are probably the same thing and come from the degenerating synergid. The antipodals are the most cytologically active cells of the megagametophyte. They have some features which are characteristic of transfer cells and possibly function in the transference of materials into the megagametophyte. Other studies (Brink and Cooper 1944) have indicated that grass antipodals are involved in the control of endosperm development. The active cytoplasm of the antipodals may reflect the synthesis or transference of growth-controlling substances.
Variation of sapwood thickness, in terms of a linear measurement (sapwood width) and a growth ring count (sapwood ring), in relation to age, height, aspect, and radial growth rate was studied in jack pine (Pinusbanksiana Lamb.) and tamarack (Larixlaricina (Du Roi) K. Koch). In general, jack pine has more sapwood rings and a greater sapwood width than tamarack. In jack pine, the number of sapwood rings steadily declined with increasing height, but in tamarack, the number of sapwood rings at first increased and then declined with increasing height. Sapwood width tended to show a species-specific constant thickness along the trunk, but both species exhibited a slight increase at the base and at the crown. The number of sapwood rings shows strong correlation with age, height, and sapwood radial growth rate, but not with sapwood width. In both species, the south aspect of the tree has wider sapwood and fewer sapwood rings than the north aspect. There is no statistical relationship between sapwood width and the number of sapwood rings.
This is a study of floret development and embryology of Stipa lemmonii (Vasey) Scribn. as a means of assessing its relationships and evolution. There is an advantage to using developmental data for it allows one to break complex structures down into developmental units. These developmental units can be compared and thus more characters can be analyzed. The lemma, palea, ventral lodicule, and carpel of S. lemmonii are leaf-like (sensu Sattler 1966, 1967) in initiation and early development. The stamens are stem-like and the ovule develops from the floret apical meristem. The ovule is interpreted as being cauline. The ovule is hemianatropous and pseudocrassinucellate. The inner integument delimits the micropyle. The integuments are ephemeral except for the inner layer of the inner integument. Embryo sac development is Polygonum type. Both synergids undergo change before entry of the pollen tube and the pollen tube appears to empty into the larger synergid. The antipodals are proliferated. The endosperm is at first free nuclear. Developmental features of S. lemmonii were compared with developmental features of S. tortilis, Oryzopsis miliacea, and S. hendersonii. The last is a species which hybridizes with S. lemmonii. Thirty-five characters were abstracted from the developmental data and were analyzed statistically. The results indicated that S. lemmonii and S. hendersonii are most closely related, and that these two plus S. tortilis form a group distinct from O. miliacea. The individual differences between taxa show gradation in the degree of difference. This is taken to indicate that the evolution of differences, and the evolution of taxa, is the result of the accumulation of many minor differences. Developmentally, plants are very simple, especially as compared with higher animals. Because of this, it is difficult to make generalizations about the "basic nature" of a plant structure using developmental data. Flowers seem best interpreted as functional units. This allows one to accept the concept that flowers in general are polyphyletic, a concept that is indicated by objective analysis of the data.
The ovules of Stipa tortilis and Oryzopsis miliacea are hemianatropous, bitegmetic, and pseudocrassinucellate (sensu Davis 1966). The hemianatropous shape of the ovule is the result of characteristic patterns of cell division and enlargement in the chalazal area and areas alongside the embryo sac. Embryo sac development in both is Polygonum-type and both have proliferating antipodals. Endosperm is nuclear, although in O. miliacea it is atypical in that nuclear division is synchronous within one portion of the embryo sac, e.g. micropylar, but not synchronous between different portions of the embryo sac, e.g., micropylar and chalazal. Differences in ovule initiation, persistence of the outer integument, fate of the inner integument, nature of the nucellus, shape of the embryo sac, nature of the synergids, cytoplasm of the egg, polar nuclei, and endosperm exist between these two taxa. Both synergids of O. miliacea undergo changes before fertilization and one degenerates before fertilization. The pollen tube enters the embryo sac at the base of the persistent synergid. There is presently insufficient embryological data to permit meaningful speculation on relationships between Stipa and Oryzopsis. Embryologically, Stipa and Oryzopsis are festucoid grasses, as much other evidence indicates. Embryo sac development in the Gramineae is more similar to that of the Restionaceae than to that of the Cyperaceae. This is in contradiction to recent speculations on the relationships of the Gramineae.
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