Parasitic Flowering Plants

Chapter 2. Hemiparasitic Santalales

Hemiparasites are green plants which have re- close relationships between taxa possessing that tained the ability to perform photosynthesis. particular sequence of genes. In such cases, the Thereby, they can supply themselves with car- relationships based upon classical morphologi- bohydrates, although not always in sufficient cal similarities in floral and vegetative structures amounts. In general, they get most of their nutri- are often overruled by the gene-molecular re- ent supply through the haustorial attachments to sults. Since not every sequence of genes has a one or several host plants. However, in periods clear expression in the visual appearance of the of their lifecycle some hemiparasites may live plant, the gene-molecular based conclusions on independent of a host. Most hemiparasitic root phylogenetic relationships sometimes make no parasites may even survive for a longer period sense in practical botany. A recent example of without a host, although they will not perform this is the inclusion of the holoparasitic Balano- very well. Typically, plant size, and flower and phoraceae in the hemiparasitic order Santalales. fruit production will be reduced. In this book a moderate, conservative view on The green colour of hemiparasites is the ob- plant classification is maintained and the follow- vious reason why we do not always recognize ing eight families are included in the hemipara- them as being hemiparasites, unless, as in the sitic order Santalales: Olacaceae, Schoepfiaceae, case of mistletoes, they are at the same time Opiliaceae, Misodendraceae, Eremolepidaceae, growing on stems and branches of other plants Santalaceae, Loranthaceae, and Viscaceae. (Fig. 21). About nine tenths of all parasitic flow- ering plants are hemiparasitic and it is among these parasites we find those which have the Olacaceae greatest influence on human life whatever the point of view is folkloristic or economic. This This family, considered the most primitive in the Chapter deals with general biology of the hemi- Santalales, has recently undergone major chang- parasitic families while haustorial development, es. It has never been clear how many out of 28 physiology, and economic importance etc. are genera included in Olacaceae are in reality para- discussed in Chapters 5-7. sitic since at least one genus Heisteria was said The phylogenetic relationships of plant spe- to be an autophyte*. Based on gene-molecular in- cies, genera and families is under constant re- vestigations, a new family, Strombosiaceae, has vision and many changes have occurred in the been established including 3 apparently non- parasitic plant families during the most recent parasitic genera from the old Olacaceae, while years based on gene-molecular studies. If a se- the revised Olacaceae contains only 4 parasitic quence of genes is conservative in the sense that genera. The name Strombosiaceae has been used the genes have a low mutation rate, it may show before and several synonyms exist, including 25 Parasitic Flowering Plants

Fig. 22. Distribution of Olacaceae. - Redrawn from http://seedplants.org/ - (About maps, see Glossary).

Erythropalaceae. Furthermore, the genus Schoe- arid and semiarid areas. The wood is very hard pfia has moved around several times and now has and dense (about 0.9 g/cm3) and used as firewood its own family. and in some cases as a substitute for sandalwood Olacaceae consists of shrubs and trees or lia- (p. 0). Both the fruits, leaves, bark and roots nas with a tropical-subtropical distribution (Fig. are used for a number of medicinal purposes. The 22). VA-type* mycorrhiza are observed in sev- seed oil is also used to soften leather. In some ar- eral genera as in the genera now separated from eas like Australia the fruit is yellow. Ximenia will Olacaceae in Strombosiaceae, while mycorrhiza germinate without a nearby host but grows much otherwise are less common in parasitic than in better when haustoria become established. other plants. The leaves are simple and most of- Another species deserves special mention. Olax ten in two rows (Fig. 24A). The flowers are small aphylla is a shrub up to 2 m high of sandy areas in (Figs. 23A-B and 24A), often 4-merous (3-6). northern Australia. The leaves are reduced to tiny Generally both sexes are present, and the ovary scales and the green stems have taken over pho- is superior to semi-inferior. The fruits have only tosynthesis. When leaf surfaces are strongly re- one seed and a thin skin like a plum. They are the duced, the plant is more able to regulate and mini- largest fruits in Santalales and 3 cm long in Xi- mize water loss through transpiration. Hence, the menia americana (hog-plum or sour plum), and plant becomes better adapted to grow under con- yellow or red when ripe (Fig. 23C). ditions where water supply is limited, as it is often Ximenia americana is unusual by having male the case in arid and semiarid areas. Such so-called and female plants. It is a pan-tropical species and xeromorphic plants are relatively rare among par- in several African countries it is also planted in asites. See also O. scandens Fig. 467. 26