Предмет: Биология, автор: Аноним

                           Особенности размножения пресноводной гидры
Способ размножения       / Условия окружающей среды/ Биологическое значение
                                       /                                              /
                                       /                                              /  

Ответы

Автор ответа: AnutcaErmolenko
0
Бесполое размножение почкованием. Гидра размножается бесполым и половым способами. Летом на теле гидры появляется маленький бугорок - выпячивание стенки ее тела17 . Бугорок этот растет, вытягивается. На его конце появляются щупальца, а между ними прорывается рот. Так развивается молодая гидра, которая первое время остается соединенной с материнской при помощи стебелька. Внешне все это напоминает развитие побега растения из почки (отсюда и название этого явления - почкование). Когда маленькая гидра подрастет, она отделяется от материнского организма и начинает жить самостоятельно. 
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Problems related to the recovery of historical elements, buildings or parts of them, draw attention to which should be the main criteria to be taken as a basis for planning
interventions. Is it correct to think of replacing parts with new ones, preserving or
integrating them? And how could they relate the preexisting?
In the restoration of historical timber as in historical building, actually, the main criteria to follow should be the conservation. Some general values concerning the building such as the original design concept or the construction techniques are cultural values to be preserved especially for monument or building with recognized artistic value.
The maintenance of such values has become a topic of considerable interest, so much that
the focus of the regulations has gradually moved towards this area. This also was made
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century that caused extensive damage, often associated with the use of inappropriate
materials of which designers and contractors did not yet know the full extent of use.
Many damages have been done between the seventies and eighties by the use of resins on stones and sculptures without any prior verification or survey; resins that are now difficult or in some cases impossible to remove. These issues introduce concepts of reversibility and compatibility of materials that today can not be underestimated.
The mandatory regulations have introduced the concepts of preservation following this direction and have formulated criteria for intervention that highlight the survey phases of the building, giving them the necessary importance.
Is worth pointing that the best recovery, for the purpose of durability, is not always that of the act if the necessary preliminary studies established the possibility of maintaining the "status quo" unchanged.
The issue of recovery of the wooden elements came back to this field and should be
approached with the same premises. Even in this field the initial experimentalism led to interventions that in many cases caused the lost of the original signs of the historic building or element. Just think of the widespread use of implants made with epoxy resin to rebuild heads trusses or deteriorated head beams. In these cases we have lost not only the original formal identity but have also failed criteria of material compatibility and future reversibility. The issue of the recovery of timber structures is intrinsically linked to three factors: the material, the structure and the construction details. The joint of these three elements contributes to the creation of the work itself, which can be found used together in structural wooden floors or roof structures such as trusses.
While in many traditions of northern Europe and of overseas countries such as United
States or Canada, wood fully contributes to the definition of the building as it is present both in vertical and horizontal structures, in Italy, where the traditional use of the masonries, beginning from the Roman period, is of prime importance; the wood has always been mainly used for horizontal structures as floors or roofs. This is especially true as one considers monumental buildings or with artistic value where significant structural capacity of wood, related to the specific weight, have always been used with success.
A frequent occasion for upgrading timber structures is now offered by the need of seismic strengthening. In case of wooden roofs, with or without wooden trusses, the problem of the intervention on the nodes is considerably complex, both from a structural and formal point of view especially considering the future reversibility. Even in the context of the
recovery of wooden structures one may unfortunately notes that the above experimentalism of the seventies, eighties led to the loss of a significant cultural heritage due to complete replace of deteriorated elements. The complete replacement of parts of the wooden
elements was due to several factors: the shortcomings of laws, lower cost of operation due to substitution with respect to the restoration but especially the lack of knowledge, sensitivity and cultural heritage of designers and owners.
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