Weed Anatomy, First Edition. Hansjoerg Kraehmer and Peter Baur. © 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

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Chapter 38

Fruit

The fruit of angiosperms has been defined as a flower at the stage of seed maturation by Knoll (1939). This definition may be criticised as being not very precise because seed maturation is not characterised by mor-phological changes alone. It makes clear, however, that the term fruit should not be associated with only seeds or the gynoecium. The onset of seed develop-ment – triggered by the fertilisation of the egg or by apomixis – may be regarded as the start of fruit devel-opment. The term angiosperm indicates that the ‘sperm’ or seed is enclosed by carpels. The carpel protects the ovule and the developing seed. It provides a surface – the stigma – on which the pollen can adhere and germinate (Friis and Endress 1990). There are, however, numerous fruits that mature without seed development. This phe-nomenon is called parthenocarpy. Seedless crops such as banana, citrus and some grape varieties are well known examples. Esau (1977) defines the fruit as a matured ovary, which makes it independent from seeds. This definition is amazingly old. According to Roth (1977), its originator is the botanist Joseph Gaertner and it dates back to the year 1788. The ovary, however, is not the only part contributing to fruit formation in many plants. Sometimes the pedun-cle and the receptacle may be involved. The carpels change their shape and consistency during fruit devel-opment. The status of a fruit also varies during its development. So, a fruit is the maturing ovary with the potential inclusion of accessory flower parts.

Fruits are called simple when they develop from one pistil. This pistil may be formed from one or several fused carpels: for example pods of legumi-nous weeds (one carpel), siliques of cruciferous plants (two carpels), berries of nightshades (two to five carpels) (Figures 38.1, 38.2, 38.3 and 38.4). An aggregate fruit consists of several pistils, such as the fruit of blackberries or that of Adonis species (Figure 38.5); the carpels are not united in this fruit form. A cup fruit is embedded in non-carpellary tissue. A free fruit is formed by a superior ovary, superior meaning that it is located at the highest level of the floral axis (Fahn 1990). Another classification of fruits is based on the way they release seed. Dry fruits, for example, can be dehiscent – that is they open – or they can be indehiscent and seeds remain within the fruit. There are further ways to classify fruits and these are described, for example, by Roth (1977) and Simpson (2010). Not every view of clas-sification in such textbooks is shared by all scientists. Even the well-accepted view that the siliques of Brassicaceae consist of two carpels is questioned from time to time (Roth 1977).

We will discuss the fruit and seed formation in weeds in more detail as the morphological character-istics of these weed elements have been identified in the literature as paramount principles of weediness (e.g. by Radosevich et al. 2007).

Ye shall know them by their fruits

171Chapter 38 Fruit

Figure 38.1Various types of fruits: (A) Rumex obtusifolius; (B) Capsella bursa-pastoris; (C) Linaria vulgaris; (D) Solanum nigrum.

(A) (B)

(C) (D)

172 Section 5 Complex Tissues and Organs

Figure 38.2Longitudinally sliced pod of Senna obtusifolia (L.) H.S. Irwin & Barneby (Syn.: Cassia obtusifolia L.) with developing seeds – one carpel!

Embryo

Carpel

Figure 38.3Longitudinally opened silique of Raphanus raphanistrum L. with developing seeds – two carpels!

CarpelPeduncle

Seeds

Figure 38.4Longitudinal section through a berry of Solanum nigum L.

Placenta

Embryo

Carpel

Figure 38.5Aggregate fruit of Adonis annua L.