502 



GARDENERS' CHRONICLE 



Plant Partnerships 



WILLARD N. CLUTE 



THE ordinary green plant is an independent individ- 

 ual ; given only light, warmth, air and a small 

 amount of water, it is able to make for itself all 

 the long list of starches, sugars, proteins, alkaloids, acids, 

 gums, tannins and other products of the vegetable king- 

 dom. A considerable number of plants, however, have 

 abandoned such an existence in favor of partnerships — 

 sometimes with other plants, sometimes with insects and 

 sometimes with other animals — apparently for the express 

 ptirpose of advancing the interests of each. To such an 

 association the term of symbiosis is given. 



There are a great many one-sided associations of 

 plants in Nature in which one s]3ecies thrives at the ex- 

 pense of the other, as in the case of the mistletoe, the 

 dodder and other parasites, and their hosts. Some stu- 

 dents would class these also as examples of symbiosi, 

 calling it an antagonistic symbiosis, but this seems stretch- 

 ing the definition a trifle. In still other cases two species 

 may be almost constantly associated, and yet neither de- 

 rive any special benefit from the jiresence of the other, 

 their OcciU'rence together being explained by the fact that 

 they obtain their food in essentially the same places. The 

 mosses that grow on trees are good instances. Such as- 

 sociations, especially among animals, are sometimes 

 known as commcnsalisni. 



The most remarkable instances of symbiosis to be 

 found in Nature are the organisms known as lichens. In 

 these the association is so intimate that what is essentially 

 a new type of plant is produced. Up to comparatively 

 recent times lichens were thought to be distinct species; 

 now they are known to consist of a fungus and an alga 

 in partnership. The lichens themselves are familiar to 

 all as the grayish-green, scaly or tufted growths on the 

 trunks of trees, old fences, stone walls and in other places 

 where the soil is too thin to support other forms of vege- 

 tation. 



In the places where lichens grow there is usually mois- 

 ture only for short periods. Certain kinds of alg;e can 

 live in such places, but they must remain dormant when 

 the water supply runs short. By combining with a 

 fungus, however,'the latter can absorb and hold the water 

 for a considerable time, and thus the algae can continue 

 to vegetate. In return for this storage of water, by 

 the fungus, the alga? provided it with food. In this way 

 both partners can thrive in a locality where either would 

 find it difficult to exist alone. 



It is not at all likely that this partnership is voluntary 

 on the part of the alga;. It has been shown that these 

 plants are held in a sort of bondage by the fungus, and 

 when removed from the lichen and cultivated by them- 

 selves, are able to grow vigorously. The fungus, treated 

 in this manner, must, of course, die for want of food. It 

 must be said, however, that the algx in the lichen seem 

 in no wav harmed by the partnership and carry on exist- 

 ence much as they do when free, possibly finding the pro- 

 tection of the fungus of some advantage. The idea that 

 the alg.T are in a sort of captivity arises from the fact 

 that new lichens can be made artificially by bringing ger- 

 minating fungus spores into contact with free algje cells. 

 The fungus threads soon surround the algae. 



An iriteresting illustration of the water absorbing 

 powers of the lichen is seen in the reindeer-moss, a com- 

 mon form in sand barrens and other sterile soils. On a 

 sunny day the lichen is so brittle that it crumbles almost 

 at a touch, but as evening approaches it gathers moisture 



from the air and becomes as soft and flexible as velvet. 

 It thus forms a very dependable natural hygrometer, by 

 which one can judge of the amount of moisture in the air. 

 For some time after the sun has risen the lichen continues 

 moist and flottrishing, and the algae, of course, are able 

 to continue food making. 



The association of the legumes or pod-bearing plants 

 with the smallest of living things, the bacteria, is regarded 

 as another form of symbiosis. In this the bacteria form 

 nodules on the roots of the host plant in which they fix 

 atmospheric nitrogen in a form that the legume can use, 

 receiving in return carbohydrates manufactured bv their 

 host. In this way the legumes, like the algae and fungi 

 in the lichen, can live in places from which they would 

 otherwise be barred. They are thus frequent in sandy 

 and sterile soils. Having their own sources of nitrogen, 

 they are not as dependent upon rich soils as other plants 

 are. This explains the fact that beans may be readily 

 grown on poor land. 



Lo ig before the Christian Era. it was known that legu- 

 minous plants, plowed under, in some way enriched the 

 soil, but it was not until late in the last century that 

 botanists discovered the reason for it. It is likely that 

 through associations of this kind in past ages most of 

 the nitrogen in the soil has been accumulated. This ele- 

 ment does not weather out of the rocks 9S other soil in- 

 gredients do. 



There is supposed to be only one species of bacteria that 

 forms partnerships with legmnes, but 'from the fact that 

 the bacteria found on the roots of one species often will 

 not grow on the roots of another, it is inferred that there 

 are numerous races of the bacterium whose scientific 

 name is Pseud omonas radicicola. Some of these races 

 will grow on more than one species, and others by special 

 cultivation may be made to do so. The bacteria of sweet 

 clover will grow on alfalfa, and that on the partridge-pea 

 will grow on cow-peas. The special race of bacteria for 

 each crop is transported from ])lace to place, clinging to 

 the seeds of the legumes, but if the soil in which legumes 

 are planted should b.appen to lack bacteria the field may be 

 inoculated with soil from a field in which a similar crop 

 has grown recently. 



Another and less well-known form of symbiosis is that 

 in which, the thready plant-body or mycelium of a fungus 

 is associated with the roots of woody ])lants. This pene- 

 trates the cortex of the root at some distance behind the 

 growing tip. and spreading out into the surrounding soil, 

 secures nitrogenous food for the olant, acting in this way 

 much like root hairs. Such fungi are called mycorrhizas. 

 Thev are ^■erv connuon in bog plants, especially among 

 the cranberries, azaleas, huckleberries and other members 

 of the heath family, but they are al$o known in oaks, 

 beeches, birches and many others. It has been suggested 

 that possibly a majority of our trees will be found to 

 have mycorrhizas when carefully studied. In the light 

 of what has just been said, it is easy to understand why 

 certain species are difficult to transplant. In digging them 

 up we often destroy the fungal symbiont, or leave it be- 

 hind when we carry away the plant. The fungi that 

 form these partnerships with the roots of trees are not 

 very well known, though the mycelia of the geasters, or 

 earth stars, .some of the shelf fungi, and various species 

 of mushrooms have been identified in such combinations. 

 Some of the ferns are also known to harbor mycorrhizas. 



