io6 



NATURE 



[y^nne ii, 1874 



which the family characterislics are so well marl<ed that 

 in no individual member of it can the signs of original 

 relationship be mistaken. 



In speaking of original relationship, I refer rather to 

 that of descent or ancestry than to community of parent- 

 age. Thus in this order we have distinct evidence that in 

 the Drosophyllunis, Droseras, nion;cas, which constitute 

 the family, the peculiarities which they have in common 

 and by which they are distinguished from other plants 

 are not possessed by them in equal development and 

 completeness, so that here as elsewhere the more developed 

 forms stand to the less perfect ones rather in the relation 

 of descendants than in that of cousins. 



In the Droseracea? the most striking peculiarity is one 

 which is entirely functional or even teleological. It consists 

 in this, that each member of it possesses in one way or other 

 adaptedness to one and the same end. This end is the 

 catching of insects, and not only catching them but digest- 

 ing them, using them as food in short, just as animals do. 

 These animal endowments, which have for some years 

 engaged the attention of our great naturalist, are pos- 

 sessed (as we hope he will some day show usj by each 

 individual species in a degree which, in the main, corre- 

 sponds to the general development of the plant ; so that 

 each advance from less to more perfect form and struc- 

 ture is accompanied by an improvement in its adapted- 

 ness to the function of preying upon insects. 



Description of titc Pldiit. — Of root and flowers I need 

 say little or nothing. It is the leaf to which 1 have to 

 ask your attention. It is of very peculiar form. The 

 blade of the leaf consists of two nearly semicircular halves 

 or lobes, which are united together along their straight 

 borders by a strong mid-rib. On to this the two lobes 

 are set in planes which are nearly at right angles to each 

 other. The curved outer edge of each lobe is strength- 

 ened by a thickened border or hem. From the hem 

 spring some twenty spikes on either side, which are 

 directed upwards and mwards. The under surface is 

 bright green, smooth and glistening, and is marked wiih 

 parallel streaks. The upper surface is pink or red, and is 

 beset with little red projections, which are called glands. 



In addition to these glands there are on the upjier sur- 

 face of each lobe of the leaf three spines, which are of 

 extreme delicacy and are always arranged as if at the 

 angles of a triangle, about the middle of the lobe. The 

 petiole or leafstalk is of the shape of the handle of a 

 lea-spoon, the only dilTercnce being that its upper surface 

 is channelled along the middle instead of being flat. At 

 iis end it is united to the leaf by a jointed isthmus, of 

 about a line in length and breadth 



The mechanism by which the leaf catches insects is 

 strikingly like that of a rat trap. When it is open the 

 lobes are, as I have said, at right angles to each other. 

 When an insect comes ii to contact with either, at once 

 they approach each other, but this does not occur with 

 the suddenness and conip'eteness that it occurs in the 

 rat trap. The lobes begm to close sharply enough, but 

 do not come quite together, remaining for some time 

 eti/r'oii-c'frt. When the leaf is in this state of half closure, 

 it is easy to see what is the significance of the two sets of 

 prongs already m.entioned. You see that they are set on 

 ahernately along the opposite edges of the lobes, so 

 that just like the teeth of the rat trap they fit into 

 each oiher. It is not difficult to see why this is, i.e. why 

 the s;jikes are arranged alternately. The leaf, being a 

 trap, is made like a trap. liut I should not have been 

 able to tell you why the leaf does not at once close on its 

 prey had not Mr. Darwin told me. After having par- 

 tially closed, as I have said, one of two things may 

 happen. The insect, having been caught, at once 

 begins to think of escaping, and makes efforts to do 

 so, which may or may not be successful. If it is 

 small, it easily finds its way out through this 

 wonderful grating formed by the crossing of the teeth ; and 



in this case the leaf soon recovers, expands again, and is 

 I ready for the capture of another victim. If it is large 

 I all its efforts to regain its liberty are futile. Repelled by 

 its prison bars, it is driven back upon the sensitive hairs, 

 which stick into the interior of its cell, and again irritates 

 them. By doing so, it occasions a second and more 

 vigorous contraction of the lobes. The resuU is that the 

 creature is not only captured, but crushed ; not only 

 swallowed, but, as I have already said, digested. 



In all this we see a wonderful completeness of adapta- 

 tion for a purpose ; but I fancy that the purpose itself 

 would be considered unworthy or even immoral by 

 I some persons. Just as in the "gentle craft" the 

 small fry are rejected and thrown back again into the 

 water to enjoy a little more life and to be better prepared 

 for their future destiny, so the plant, not quite for the same 

 reason, acts in a similar manner. The angler rejects the 

 small fish with a view to their future and his own, for he 

 wants them to grow larger that he may have the better 

 sport out of them afterwards ; but the plant lets the little 

 insects go, because it would cost too much to keep them ; 

 ' and this leads me to the description of what happens to 

 the leaf and to the poor fly when it is big enough for the 

 leaf to find it worth while capturing, /<•. when it is too big 

 to slip through the bars. 



Digt'stio?! of Uwnaa. — Even after slight irritation, such 

 I as that which is produced when a fly merely touches one 

 i of the sensitive hairs, or when they arc touched with 

 a dry camel-hair pencil, the leaf remains closed for some 

 time, usually more than twenty-four hours. But if a fly 

 is caught, or any other nutritious substance is introduced, 

 the case is differenf. For a week or more the leaf re- 

 mains closed on its prey, the two lobes being at first 

 pressed flat against each other. The two lobes indeed 

 close round the fly so completely that its body gives rise 

 to two projections of the (outer) surface of each lobe, 

 which correspond to it in form. The result of this is that 

 the secreting glands on the part of the leaf against which 

 the body of the fly presses are irritated, and begin to 

 pour out a quantity of secretion. (Gradually this effect 

 extends to the rest of the leaf, and consequently its caxity 

 becomes gradually extended. 



The meaning of this bulging is that the fly is becoming 

 digested. The liquid juice which the glands pour ouc 

 has the property of so acting on the tissue of the fly's 

 body that they at first become diffluent and then are 

 absorbed. 



When we call this process "digestion " we have a defi- 

 nite meaning. We mean that it is of the same nature as 

 that by which we ourselves, and the higher animals in 

 general, convert the food they have swallowed into a form 

 and condition suitable to be absorbed, and thus available 

 for the maintenance of bodily life. 



The nature of animal digestion is best explained by 

 examples. If I take some starch, which is not soluble, 

 and put it into my mouth, and keep it there for a certain 

 lime, it has become first soluble, and finally transformed 

 into a substance quite different in properties. If we ex- 

 amine into this process we find that the change of starch 

 into sugar takes place, because there exists in saliva a 

 ferment called ptyaline. We know that it is the ptyaline 

 which does the work, because if we separate this sub- 

 stance in a solid state, then dissolve it in water in which 

 starch is diffused, the starch is converted into sugar. 

 We call it a ferment, for two reasons — first, because, 

 like leaven, it acts in small quantity, a mere trace being 

 sufficient ; and secondly, because it does not itself take 

 part in the transformation. This is one example, and a 

 very simple one ; but it is not wuh this that we compare the 

 digestionof Dionaja,butwith that which in man and animals 

 we call digestion juoper, the process by whch the nitro- 

 genous constituents of food are rendered fit for absorption. 

 This takes place, not in the mouth, but iii the stomach. 

 It also is a fermentation, /.<■. a chemical change effected 



