The Acacia Plant

The Leaf | The Inflorescence | Pollination | Seeds | Growth

Development of the seeds

Following pollination and fertilisation the ovary develops into the legume containing a number of seeds. The seeds are attached to the legume by a stalk, called a funicle, which usually expands into an aril at its point of attachment to the seed. The seed coat, or testa, is hard and resistant to moisture penetration.

The legumes, at least of most Australian species, are dehiscent, and split open along one suture. The seeds may be ejected from the legume when it opens elastically under the influence of the hot sun. In a few other cases, the seeds may sometimes be shaken from the open legume by the wind, or they may be dispersed by birds. In this case, the seeds usually hang from the open pod by their often red, yellow or orange coloured aril and funicle, making them highly visible to the birds. Particularly in more arid areas ants are frequently a secondary dispersal agency, removing the fallen seed, consuming the fleshy aril and at the same time burying the seed²¹. Some Acacia legumes do not readily split open (indehiscent), such as A. farnesiana. In the case of similar Africian species, the legumes are consumed by large herbivores and the indigestible seeds are excreted⁷⁷. In Australia, the role of our now extinct herbivore megafauna on the native flora can only be conjectured, but domestic stock may be a major cause of the spread of A. farnesiana since European settlement. The dry seeds of this species have a hard, thick testa requiring a load inexcess of 24 kg to fracture it^26,77.

An external agency, such as the heat from a bush fire or high summer temperature, or abrasion from wind-blown sand or other agency, is required to fracture the testa so that moisture and oxygen can penetrate and initiate germination. However, this 'hardseedness' varies, and a small percentage of seed is frequently 'soft' and will germinate readily without the necessity for the testa to be fractured. This variation in hardseedness of a batch of Acacia seed is probably a survival strategy. Some seed will be stimulated to germinate readily but may then fail due to lack of adequate follow-up rains. The remainder of the seed will germinate over varying lengths of time as adequate moisture and oxygen penetrate through the testa to the seed with the increased possibility of the survival of the seedling.

The Leaf | The Inflorescence | Pollination | Seeds | Growth