July 27, 1888.J 



SCIENTIFIC NEWS. 



85 



FRUITS. 



Considered from a Darwinian Point of View. 



A STUDY of some of the common fruits that we 

 eat at table gives us some excellent examples 

 of the Darwinian theory of natural selection and the 

 survival of the fittest. 



Let us begin by comparing the botanical definition 

 of a fruit with the structures popularly known under 

 the term, " fruits." A fruit, as botanically defined, 

 is simply the matured ovary of the plant after fer- 

 tilisation has taken place ; but in the case of many 

 of the fruits we eat at table other parts of the flower 

 are included. 



The simplest form ol fruit is simply a case en- 

 closing a single seed ; such a fruit is called an 

 qchene. The buttercup is a good example of this 

 form of fruit, and each of the individual fruits of the 

 strawberry is an achetie. The seed case is known as the 

 pericarp, and usually consists of three layers : an outer — 

 the epicarp ; a middle — the mesocarp ; and an inner — the 

 endocarp. 



The simplest form of succulent fruit is the drupe. In 

 this the seed case has become succulent and fleshy, and 

 its three component parts are distinguishable, and have 

 taken on different functions, for reasons we shall after- 

 wards see. The epicarp forms an external, usually highly 

 coloured skin; the mesocarp forms the succulent matter; 

 while the endocarp forms a hard, protective covering for 

 the seed. Examples of this form of fruit are the cherry, 

 plum, etc. The blackberry and raspberry are formed by 

 an aggregation of such drupes placed on the enlarged re- 

 ceptacle. These are known as compound drupes, and each 

 separate fruit is a drupel, each drupel having, of course, 

 the same structure as the cherry. 



Such fruits as the above are true fruits, but let us 

 now notice a few well-known examples in which other 

 parts of the flower go to form the so called " fruit." These 

 are spurious fruits. In the first place, let us take the straw- 

 berry. In this the red, swollen mass is simply the end of 

 the stalk, which has become succulent, while the fruits 

 themselves are the yellow bodies in the surface, which are 

 commonly called seeds, each ol these fruits, as we have 

 seen before, being an achetie. Again, the end of the 

 stalk, or receptacle, instead ol bulging out, as in the 

 strawberry, may become hollowed out so as to enclose 

 a cavity in which are placed the fruits. The fig is 

 an example of this. The fig is thus a strawberry 

 which has become, so to speak, invaginated. The 

 type of fruit termed a berry is a succulent fruit in 

 which the seeds are imbedded in a pulpy mass — e.g., 

 the gooseberry and grape. The blackberry, raspberry, 

 and strawberry are therefore not true berries at all. 

 Now let us take the apple. This is a still more modified 

 form, as the calyx tube, in addition to the pericarp, en- 

 closes the seeds. The calyx tube has become fleshy and 

 forms most of what we eat; the mesocarp also is fleshy; 

 while the epicarp and endocarp form two horny cases. 

 Finally, the most modified fruit is the mulberry. Here 

 the group of flowers, the petals of which have 

 become succulent, forms the so-called mulberry fruit. 

 The pineapple is another example of this kind. 



These few familiar examples will suffice to show what 

 different parts of the flower go to form what are popu- 

 larly known as fruits, and also what different parts may 

 become succulent. In the cherry, blackberry, raspberry, 

 etc., it is the true seed case which becomes succulent ; 



in the strawberry it is the swollen stalk ; in the apple 

 the calyx tube ; and in the mulberry the petals of the 

 flowers themselves. 



Now let us consider the reason why some fruits are 

 succulent and others hard ; why some are sweet and 

 others bitter ; and why some are highly coloured and 

 others dull. The primary object of all fruits is the per- 

 petuation of their species, by means of dispersing their 

 seeds. It is obviously of advantage for the seeds to be 

 scattered over as wide an area as possible, because only 

 a certain number of plants can grow in a certain area. 

 The seeds have no inherent power of locomotion ; hence 

 they must avail themselves of such opportunities as occur. 

 The simplest agent in dispersal is the wind, and seeds 

 which avail themselves of this means are either minute, 

 so as to be easily blown about, or else they are downy, 

 as in the dandelion, so that they can float a long way in 

 the air. The second means of dispersal is by animals, 

 chiefly birds, and such seeds avail themselves of this 

 means which cannot be dispersed by the wind. Ani- 

 mals may cause dispersion of seeds in two ways. Firstly, 

 by carrying earth with seeds imbedded in it in their 

 claws. A good example of this is given by Darwin, who 

 states that he obtained a piece of earth which had been 

 found sticking to the leg of a partridge, and since then 

 had been kept three years, but when wetted and placed 

 under a bell-jar, he obtained no less than eighty-two 

 plants from the seeds imbedded in it. This also shows 

 the length of time that seeds retain their power of germi- 

 nation. Secondly, animals may disperse seeds by 

 swallowing them as food. The seeds of such fruits are 

 protected by a hard coat, which resists the action of the 

 digestive fluids of the animal's stomach, and the seeds 

 thus pass out in the excrement uninjured, and get dis- 

 persed during the peregrinations of the animal. Fruits 

 which attain the dispersal of their seeds in this way 

 become as succulent and sweet as possible, and acquire 

 bright colours to attract the birds and other animals ; but 

 this does not take place till they are ripe, for otherwise 

 they would be eaten before the seeds were mature. In 

 other words, such fruits strive as hard as possible to get 

 eaten. Take, for example, the wild strawberry ; the 

 young strawberry is green and sour, to prevent being 

 eaten till the seeds are mature ; when, however, the 

 seeds are attaining maturity, the receptacle becomes red 

 and succulent, to attract the notice of the birds ; the latter 

 swallow the pulpy receptacle and with it the small 

 yellow fruits ; the receptacle is digested, but the seeds 

 pass through uninjured and so become dispersed by the 

 flight of the bird. 



Hence we see the reason for fruits becoming succu- 

 lent and sweet is in order to make themselves attractive 

 food for birds and other animals, and so ensure the dis- 

 persal of their seeds. Such fruits we may call attractive 

 fruits. But this explanation will not account for the 

 origin of the nuts or deterrent fruits. In these it is the 

 seed itself which is nutritious, owing to the large store 

 of albuminous material destined for the young embryo 

 plant. Owing to this large store of food, these fruits do 

 not require dispersal, hence do not make themselves at- 

 tractive ; on the contrary, they make themselves as re- 

 pulsive as possible, and devote all their energies to 

 prevent getting eaten and to preserve their nutritious 

 seeds, as it is evident that these seeds, since they are 

 digestible, will cause destruction of the species if they 

 are eaten. 



These deterrent fruits may be protected by a prickly 



