It is not only bacteria which may live within plants, but some types of specialised fungi may also occupy such a niche. Their presence is fairly widespread and may be found in various grasses and a wide range of commercial crop plants including tomatoes, apples, beans, wheat and corn. One type of plant – fungus association, actinorrhizae, has already been mentioned in the section describing symbiotic nitrogen fixation. The fungal hyphae penetrate the plant cells where a variety of structures may develop such as swellings or the development of coils or small branches. Vesicles and arbuscules which are branched structures reminiscent of a tree, are common features of this invasion. Despite how this description may appear, such association of plant with fungus may be very beneficial to the plant. In exchange for energy derived from the plant through photosynthesis, the fungus may enhance the supply of available nutrients to the plant under conditions of relatively high humidity and conversely, in dry conditions, the fungus may help the plant in the uptake of water. In addition, some fungi have been found to affordprotection to the plant in cases where the fungi produce alkaloids rendering the plant less susceptible to attack by chewing insects.