Sept. 24, 1874J 



NATURE 



423 



is useful in making the flovvet conspicuous. The inner florets 

 are also tubular, but are small, yellow, and without rays. Each 

 of these florets is furnished with stamens as well as a pistil. Tlie 

 stamens are united on their inner sides so as to form a closed 

 tube, within which the pistil lies. They ripen before the pistil, 

 and dehisce on their inner sides, so that the pollen is discharged 

 into the upper end of tlie tube above the head of the pistil. 

 When the flower opens the pollen is already ripe, and fills the 

 upper part of the stamen tube. A floret in this condition is re- 

 presented in Fig. 15. The pistil, however, also continues to 

 elongate, and at length pushes the pollen against the upper end 

 of the tube, which gives way, and thus tlie pollen is forced out of 

 the tube, as shown in Fig. :6. The pistil itself terminates in two 

 branches, which at first are pressed closely to one another, and 

 each of which terminates in a brush of hairs (Fig. 17). As the 

 style elongates this brush of hairs sweeps the pollen cleanly out 

 of the tube, and it is then removed by insects. When the pistil 



y-vfc 





has attained its full length two branches open and curve down- 

 wards so as to expose the stigmatic surfaces (Fig. 17, st) which 

 had previously been pressed closely to one another, and thus 

 protected from the action of the pollen. From this anangc- 

 ment it is obvious that any insect alighting on the flower-head of 

 the Chrysanthemum would dust its under-side with the pollen of 

 the younger flowers, which then could not fail to be brought into 

 contact with the stigmatic surfaces of the older ones. As the 

 expansion of the flowers begins at the outside and thence extends 

 to the centre, it is plain that the pollen of any given floret can- 

 not be used to fertihse one situated on its inner side. Conse- 

 quently, if the outer row of florets produced pollen, it would, in 

 tlie great majority of cases, be wasted. 1 have, however, already 

 mentioned that these florets do not produce pollen, while the 

 saving thus effected enables them to produce a larger corolla. 

 It is also interesting to observe that in these outer flowers the 



branches of the pistil do not possess the terminal biusli of haiis 

 which, in the absence of pollen, would be useless. 



In other Compositae, as in the Marigold, while the ray floweii 

 produce no pollen, the disc flowers develop stigmas only. In this 

 case, as in the Feverfew, the pistil of the ray flowers does not 

 require or possess the terminal brushes of hairs, as there is no 

 pollen to be swept out. The central flowers, on the other hand, 

 tliough they develops no stigmas, require a pistil in order to 

 force the pollen out of the anther tube. Hence the pistil is pre- 

 sent as usual, but the head is simple and not bifid. This corc- 

 plete allerat'oti of the function of the pistil is extremely curioi'S 



Perhaps no group of flowers offers irore reniarkalils adap!a- 

 tions than the orchids, which have been so admirably described 

 by Mr. Ijarwin.* As an illustration of our English species, 

 I shall take the common early purple orchis (Orchis mascula), 

 as being one of the commonest, if not the commonest, species ; 

 and a fair example of some of the remainder, which however 

 differs in many interesting and important points. 



Fig. iS represents the side view of a flower, from which all 

 the petals and sepals have been removed, except the labellum (/), 

 half of which has been cut away, as well as the upper portion 

 of the near side of the nectary («). The pollen forms two massts 

 (Fig. 19, a a), each attached to a tapering stalk, which gives 

 the whole an elongated pear-like form, and is attached to a 

 round sticky disk (i/), which lies loosely in a cap-sliaped envelope 

 or rostellum (;). This envelope is at first continuous, but the 

 slightest touch causes it to rupture transversely, and thus to 

 expose the two viscid balls (Fig. 20, </). Now suppose an insect 

 visiting this flower ; it alights cm the labellum, and pushing its 

 proboscis down the nectary to the honey, it can hardly fail to 

 bring the base of the proboscis into contact with the two viscid 

 discs, which at once adhere to it, so that when the insect draws 

 back its proljoscis, it carries away the two pollen masses. It is 

 easy to imitate this with a piece of grass, and to carry away on it 

 the two pollen masses and their stalks. If, however, the poUinium 

 retained this erect position when the insect came to the next 

 flower, it would simply be pushed into or against its old position. 

 Instead however of remaining upright, the poUinia, by the con- 

 traction of the minute disc of membrane to which they are 

 attached, gradually turn downwards and forwards, and thus when 

 the insect sucks the next flower, the thick end of the club exactly 

 strikes the stigmatic surface (st). The pollinium or pollen mass 

 consists of packets of pollen grains, fastened together by elastic 

 threads. The stigma, however, is so viscid, that it pulls off 

 some of these packets, and ruptures the threads, without re- 

 moving the whole pollinium ; so that one pollinium can fertilise 

 several flowers. 



I cannot resist mentioning the case of Cataselum, one of the 

 Vaudrx, which, as Mr. Darwin says, "are the most remarkable of 

 all orchids." In Catasetum (Fig. 21) the poUinia and the stigmatic 

 surfaces are in different (lowers, hence it is certain that the former 

 must be carried to the latter by the agency of insects. The 

 pollinla, moreover, are furnished with a viscid disc^ as in orchis, 

 but from the large size of the flower, and the position of the honey, 

 the insect has no inducement to approach, and in fact does not 

 touch, the viscid disc. The flower, however, is endowed with 

 a peculiar sensitiveness, and actually throws the pollinium at the 

 insect. Mr. Darwin has been so good as to irritate one of these 

 flowers in my presence : the pollinium was thrown nearly 3 ft, 

 when it struck and adhered to the pane of a window. This 

 irritability, however, is confined to certain parts of the flower of 

 Calascluiii sacialnm, which is also shown in section in Fig. 22. 

 In this figure it will be seen that the pollinium (dp) is curved, 

 and in a state of considerable tension, but retained in that posi- 

 tion by a delicate membrane. Now, insects alight as usual on 

 the labellum of the flower (/), and it will be seen that in front of 

 it are two long processes, or antenna; (aii). In some species of 

 Catasetum both these antenna; are highly irritable; in the present 

 species the right-hand one is apparently functionless ; but the 

 moment the insect touches the left-hand one, the excitement is 

 conveyed along it, the membrane retaining the poUinium is 

 raptured, and the latter is immediately jerked out of the flower 

 by its own elasticity, with considerable force, with the viscid 

 disc foremost, and in such a direction as to come in contact with 

 the head of the insect which had touched the antenna. 



I will only mention one other tropical flower, the very curious 

 Marcgrcwia nepcnihoidcs, described by Mr. Belt in his intc- 

 restmg work, "The Naturalist in Nicaragua." The flowers 

 are disposed in a circle, and beneath them are suspended some 



* Fertilisation of Orchids. 



