Tansley Review No. 110.

Abstract

Orchid seeds are very small, extremely light and produced in great numbers. Most range in length from c. 0.05 to 6.0 mm, with the difference between the longest and shortest known seeds in the family being 120-fold. The ' widest ' seed at 0.9 mm is 90-fold wider than the ' thinnest ' one, which measures 0.01 mm (because orchid seeds are tubular or balloon-like, ' wide ' and ' thin ' actually refer to diameter). Known seed weights extend from 0.31 µg to 24 µg (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4 000 000 and as low as 20-50 (80 000-200 000-fold difference). Testae are usually transparent, with outer cell walls that may be smooth or reticulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller : their volume is substantially smaller than that of the testa. As a result, orchid seeds have large internal air spaces that render them balloon-like. They can float in the air for long periods, a property that facilitates long-distance dispersal. The difficult-to-wet outer surfaces of the testa and large internal air spaces enable the seeds to float on water for prolonged periods. This facilitates distribution through tree effluates and\or small run-off rivulets that may follow rains. Due to their size and characteristics, orchid seeds may also be transported in and on land animals and birds (in fur, feathers or hair, mud on feet, and perhaps also following ingestion).

Seed dispersal in six species of terrestrial orchids in Biebrza National Park (NE Poland)

“…The variable properties of seeds (shape, weight, proportion of air space) are among the causes of differentiated distances of seed dispersal in orchid species [38]. For example, Arditti and Ghani [38], Shimizu et al [58], and Kiyoharaet et al [59] found different seed velocities according to certain properties, such as weight or proportion of air space. Seeds with large air spaces drop more slowly, and therefore will have a lower settling velocity.…”

“…The lack of such information for one of the most endangered plant families is an important deficiency, because this parameter influences the persistence, dynamics, and evolution of orchid populations. Orchid seeds are generally assumed to be widely dispersed by wind because of their small size and low weight, in addition to their embryos occupying only a very small percentage of the space inside the testae (with an air content from 60 to above 90%) [36][37][38]. According to Gandawijaja and Arditti [39], these characteristics enabled orchids to reach Krakatau from Java (distance ca.…”

“…Besides seed properties, vegetation type, wind characteristics, and plant size are the most often considered factors influencing dispersal distance in species with dust seeds [16,[40][41][42]. Although some authors have stated that information about long-distance dispersal over more than 10-15 km should be regarded as extremely rare [43], it is often reported in orchid literature [28,33,38]. Estimations of long-distance dispersal of orchid seeds have not been appropriately documented by empirical studies and often appear to be overestimated, affecting the interpretation of factors that shape the demographic and genetic structure of orchid populations.…”

Role of Auxin in Orchid Development

“…45 The mature orchid seed is "dustlike" in size and weight, lacks a primordial shoot/root axis, and the protoderm is the only distinctive primary meristem. [46][47][48] Upon germination, embryo cells divide to form a protocorm, which is a tuberous, sphere or oblong structure that is several times the dimensions of the original embryo, and comprised of undifferentiated cells. 45,[49][50][51] Approximately two to three weeks later, a meristem forms with evidence of a first leaf.…”

Cultivo in vitro de Epidendrum nocturnum (Orchidaceae) ocorrente no Cerrado da região Centro-Oeste

“…As orquídeas possuem limitações que dificultam a germinação de sementes como, por exemplo, a espessura da parede das células do tegumento envoltório do embrião e a quantidade de endosperma presente no embrião que, muitas vezes, é insuficiente para a germinação (Arditti & Ghani 2000). As plântulas de orquídeas podem ser obtidas, em laboratório, tanto por germinação assimbiótica in vitro, utilizando meios de cultivo assépticos (Hossain et al 2010;Favetta et al 2014), quanto germinação simbiótica com fungos micorrízicos (Zettlet et al 2007) ou, ainda, micorrização de plântulas provenientes de germinação assimbiótica (Wu et al 2011).…”