238 



SCIENCE-GOSSIP. 



and hand a portion of it ov^er as a food-stuff to the 

 higher plant. The latter, in return, gives some of 

 the starchy food which its green leaves have 

 prepared to the organisms in its roots. Thus the 

 work of nutrition is, in this case, shared between 

 two plants standing at opposite extremes of the 

 vegetable kingdom. Any reference to symbiosis 

 would be incomplete without a mention of the 

 lichens. The work of Schwendener, of Stahl, of 

 Reess and of others has shown that these plants 

 are not self-standing, but rather to be regarded as 

 fungi which have grown round and enclosed within 

 them certain algas. The relation existing between 

 the two is not one of parasitism in the strict sense, 

 however, but of symbiosis. Both fungus and alga 

 have marked advantages from the combination. 

 The colourless fungus is enabled to assimilate food ; 

 and it is rendered possible for the green alga to 

 exist in situations v;hich would be impracticable to 

 it alone ; moreover, the fungoidal covering protects 

 the alga from many external dangers. 



Another somewhat similar but less certain case 

 of partnership is that which is said to exist 

 between certain animals and algae. The common 

 Hydra viridis of our ponds represents a union of 

 this kind. The body of the Hydra consists of an 

 outer columnar layer of cells — the ectoderm — and 

 an inner irregular layer — the endoderm, — between 

 the two strata being interposed a delicate 

 membrane — the mesogloea. The inner endoderm 

 cells abut upon the central cavity of the Hydra, 

 and take a prominent part in the digestive 

 processes of the organism. They are amoeboid 

 in their nature, constantly altering their shape ; 

 they are furnished each with a nucleus, a flagellum, 

 and a large vacuole. Besides these things they 

 have one other remarkable feature in that they are 

 studded with granules of green chlorophyll. Now 

 chlorophyll is the characteristic possession of the 

 plant world, and it has been asserted that where- 

 ever this green pigment occurs in the animal 

 kingdom it is present in the form of a low alga, 

 which is included in the animal and stands in 

 symbiotic relationship to it. Whether the chloro- 

 phyll grains of Hydra, Paramoecium, etc., are true 

 organs of the animal in which they occur, or 

 imprisoned algas, is a question which can by no 

 means be considered decided, although so careful 

 an observer as Brandt is responsible for the latter 

 view. 



The illustration of symbiosis furnished by 

 flowers and insects will be familiar to all. The 

 pollination of the yuccas, a class of lilies, which 

 Professor Riley has only lately (1892) explained, 

 may be worth repeating here. 



A moth, Pronubayuccasella, when about to lay its 

 eggs, flies to a newly-opened flower of yucca, 

 pierces a hole into its ovary, and deposits its eggs 

 in some six or more ovules. It then flies to the 



anthers and collects together a small mass of pollen 

 which it stuffs into the hollow stigma of the plant. 

 By all this it effects the fertilization of the flower at 

 the expense of some few of its ovules, and at the same 

 time it provides for the shelter and maintenance of 

 its young. How dependent this plant is on the 

 moth will be seen from the fact that in Philadelphia, 

 where the moth appears when the yucca blooms, 

 numerous seeds are formed, whilst in St. Louis, 

 where the moth only comes after the time of 

 flowering, no seed is developed. 



In those countries where the leaf-cutting ant 

 strips the higher plants of their foliage, a curious 

 partnership is entered into between the ordinary 

 ants and certain acacia plants. In Acacia spharo- 

 cephala, for instance, we find the thorns on the 

 stem curiously hollowed out and opening on to the 

 exterior by small holes. In these cavities the ants 

 find a safe and convenient home. Not only does 

 the plant house its visitors, but it also provides them 

 with refreshments : from its leaflets it excretes 

 small nutritious particles, which are eagerly de- 

 voured by the ants, and having taken off the edge 

 of their appetites it tickles their palates still further 

 with the nectar it secretes. Fortunate indeed are 

 these ants beyond their fellows, and well should 

 they deem their task a light one of repelling their 

 leaf-damaging relatives. These latter themselves 

 live under an even more curious symbiotic condi- 

 tion. The leaf fragments that they collect together 

 they arrange as a pabulum for a certain fungus 

 {Rozites gongylophora) which forms their staple 

 food-supply. In order that this may be always to 

 hand they plan those wonderful " fungal gardens " 

 which Moller has so well described. 



There is another case of comradeship between 

 two animals which is well worth examining : it is 

 the association of a certain hermit-crab {Paguriis 

 prideauxii) and a sea-anemone (Adamsia palliata). 

 The hermit-crabs, taken as a class, are curious 

 in the possession of soft and unprotected tail- 

 portions to their otherwise chitinous bodies. To 

 protect this vulnerable organ the majority of them 

 seek a whelk- or winkle-shell in which they can 

 thrust this soft part, and in their travels drag the 

 house they have appropriated after them. Paguyus 

 prideauxii, however, is wiser than its brethren ; in 

 its young days it attaches itself to a very small 

 shell, and to this habitat it sees that a specimen of 

 a particular sea-anemone also becomes fixed. As 

 the crab grows the shell only covers the tip of its 

 soft and now large tail, and in the ordinary course 

 of events it would have to undertake the unpleasant 

 duty of house-moving. 'But in this particular far- 

 seeing Pagurus the difficulty is obviated, for the 

 anemone which became attached to this stolen 

 house has, like the inmate, grown considerably ; 

 first it spread over the entire shell, then bit by bit 

 it grew forwards beyond this on to the body of the 



