MOVEMENTS DUE TO TURGOR AND GROWTH 427 



orchids the pollen-grains are here united into pollinia, and the pollinia are 

 connected by means of a stalk, the caudicle (st), with a cement disc (K). The space 

 relations of these three bodies may be best seen by an examination of the longi- 

 tudinal section of the column (Fig. 131, IV). The anther, it will be seen, 

 placed at the end of the long filament (/) is curved downwards, and it may be 

 noted that the caudicle uniting the pollinium and the cement disc is bent over a 

 small cushion of tissue, the rostellum ; perhaps it would be more accurate to say 

 that the caudicle is composed of the outermost cells of the rostellum and has 

 become differentiated from the deeper seated parts. The cement disc is also 

 a part of the rostellum, and only that side of it which faces backward is sticky. 



If the flower be placed under appropriate external conditions, one of these 

 especially being a suitable temperature, and if one of the antennae be lightly 

 touched with a pencil or splinter of wood, the caudicle and cement disc are 

 released from the rostellum, the caudicle straightens itself, and throws the 

 cement disc forwards with considerable force. The movement is so vigorous that 

 the entire pollinium is jerked out of the flower and is shot through the air, cement 

 disc first. If it meets any obstacle in its passage the cement disc becomes 

 firmly attached to it. The biological significance of the movement is perfectly 

 clear, it is one of the extremely interesting adaptations one meets with in 

 Orchidaceae for bringing about cross-pollination by insect agency. It is 

 unnecessary to go into biological details ; what more immediately concerns us 

 is, in the first place, the nature of the tensions in the caudicle, which is obviously 

 the mechanical cause of the movement and, further, the significance of the 

 friction applied to the antennae. 



As to the nature of the tensions in the caudicle there have been no investi- 

 gations, and absolutely nothing is known as to how the effort to elongate the 

 inside of the caudicle arises ; it may be due to swelling, osmotic pressure, or 

 growth. An investigation of the structure of the cells of the caudicle should 

 prove an acceptable subject for research at the hands of the physiological 

 anatomist. Moreover, the straightening of the caudicle longitudinally is by 

 no means the only movement which takes place ; at the same time, an inrolling 

 of its edges occur, so that the outer parts along the edge of the caudicle exhibit 

 stronger efforts to elongate than the inner parts. 



The stipe may be made to spring loose not only by rubbing the antennae 

 but also by the application of pressure to the caudicle itself. Such a pressure 

 may be compared with the shaking or bending in the slinging movements 

 previously mentioned ; obviously it leads to increased tension and so to an 

 explosion when the resistance is overcome. The touching of the antennae 

 is another matter altogether ; there we have to deal with a genuine stimulus, 

 and this stimulus is applied at a considerable distance from the place where the 

 activity is manifested. It is quite out of the question that any contact between 

 a pencil point and the antenna can possibly lead to a mechanical deformation 

 of the caudicle, and so to an increased tension in that organ. What the prece- 

 dent phenomena are, how they are transmitted, and how they lead to an ejection 

 of the pollinia is as yet quite unknown. 



Hitherto we have considered only such movements as take place in response 

 to an external stimulus applied to some part of the living plant. This stimulus 

 may be light, heat, electricity, gravity, or a mechanical or chemical action of 

 some body. We may indeed distinguish two types of action of such stimuli, viz. 

 general and special. The general stimuli or so-called formal conditions are 

 necessary in order that the plant may, first of all, be in the condition to react, in 

 order that growth and movement may be possible, and in order that special 

 stimuli may be able to induce movements in it. To the category of general 

 stimuli belong a certain degree of temperature, a certain amount of oxygen, and 

 all substances which we have termed nutrients. The special stimuli very fre 



