Diplotaxis tenuifolia is a species of flowering plant in the mustard family known by the common name perennial wall-rocket. It is native to Europe and western Asia, where it grows on disturbed ground and roadsides, and it can now be found throughout much of the temperate world where it has naturalized. In recent years it has increasingly been cultivated to produce salad leaves, which are marketed as wild rocket in Britain or arugula in the US. It is easily confused with garden rocket, which has similar uses.
Description
Perennial wall-rocket is a glabrous herb with an erect or sprawling habit, that grows up to 1.3 m tall, with a solid, almost woody terete stem and spreading branches. The deeply pinnate leaves are up to 12 cm long and often rather fleshy, with a peppery taste and a musty smell.
In the British Isles, it flowers from May to September (or through October in a warm year). In Spain, it can be seen flowering between April and December. The inflorescence is a branched raceme up to 30 cm long with up to 30 flowers, each of which has 4 free bright yellow petals up to 15 mm long, and 4 free yellow/brown sepals up to 7.5 mm long. Each flower has 6 stamens and a single style.
The fruit is a straight, flat silique (pod) up to five centimeters long. The pedicels are shorter than the fruits and ascend at an acute angle to the stem. Above the receptacle is a short (2 mm) stalk (or stipe) below the pod (a useful feature for separating this species from annual wall-rocket). The seeds are arranged in 2 rows, or staggered in a zigzag pattern, towards the centre of the pod, which in turn consists of 2 valves, so a cross-section of the fruit shows 4 seeds in total. This is a distinguishing feature of the genus Diplotaxis, although it is not always easy to see, as plants are not self-pollinating, so ripe seeds do not always develop.
Taxonomy
The original name (basionym) of perennial wall-rocket is Sisymbrium tenuifolium, published by Linnaeus in 1755, but it was transferred to the new genus Diplotaxis by de Candolle in 1821. It has been given many other names (synonyms) over the years, within the diverse genera Arabis (rockcresses), Brassica (mustards), Crucifera, Eruca (rockets), Erysimum (wallflowers) and Sinapis (charlock), which reflects the complex history of crucifer taxonomy. A full list is maintained by the Catalogue of Life Partnership.
There is only one currently recognised subspecies: Diplotaxis tenuifolia subsp. cretacea (Kotov) Sobr.-Vesp., which was described in 1996 and which is restricted to Ukraine and western Russia.
It is not known to hybridise with any other species.
The chromosome number is 2n = 22 (based on British material).
The generic name Diplotaxis comes from the Ancient Greek, διπλός (diplos) = twofold, or double; and τάσσω, τάσσειν (tasso, tassein), which means to put into order, or to draw up in a line (as with troops before a battle). It refers to the double row of seeds in each part of the fruit, which distinguishes this genus from others within the Brassicaceae. The specific epithet tenuifolia simply means "narrow-leafed".
The common name "rocket" is a corruption of the Italian word ruchetta, which describes various cruciferous plants. This species is also sometimes called wild rocket, sand rocket, Lincoln weed (mainly in Australia) and white rocket; the seeds are sometimes marketed as "wild Italian arugula" or "sylvetta arugula".
Identification
In Britain it is most likely to be confused with annual wall-rocket, but it does not have a basal rosette, the leaves are divided more than half way to the midrib, and the fruit has a short stalk (stipe) above the sepal scars.
The Diplotaxis species can be separated from most other crucifers by the double row of seeds in each valve of the fruit (although this is a difficult feature to see in unfertilised plants) and a seedless beak to the silique.
Distribution
It is native to parts of Europe, reaching in the east to Turkey and Syria. It is extinct from Morocco and Algeria, but has been introduced in a number of regions: Cyprus, the Caucasus, Palestine, Yemen, Japan, Australia, New Zealand, New Caledonia, Canada (Ontario, Quebec and Nova Scotia), the United States, Costa Rica, Argentina and Uruguay.