1881.] 



MICROSCOPICAL JOURNAL. 



12' 



of protoplasm in the plant-cell is not 

 aided by light, if, indeed, it does not 

 take place more readily in the dark. 

 It has also been shown that it cannot 

 take place without a supply of oxy- 

 gen. It is also a familiar fact that 

 the rotation of protoplasm is common 

 in plant-hairs, in which it may be seen 

 under the microscope. A favorite 

 object for this purpose used to be the 

 stinging hair of the nettle, but the 

 cyclosis is easily seen in many other 

 hairs, particularly in the segmented 

 hairs on the stamens of Tradescantia 

 and in the leaf-hairs of the pansy or of 

 the holly-hock. The facts I have just 

 referred to serve to connect the hairs 

 with the protoplasmic portion of the 

 plant and, through it, with the pro- 

 cess of metastasis. 



As there are two essential physio- 

 logical processes in plant-life, so there 

 are two principal sets of organs for 

 the effecting of these processes, name- 

 ly, the chlorophyll-cells with their 

 appendages, and the circulatory sys- 

 tem with its appendages. The for- 

 mer system is apparently much the 

 simpler, and consists of little be- 

 sides the chlorophyll-cells themselves 

 (which are parenchymal cells), with 

 the stomata, their means ' of com- 

 munication with the external air. 

 But the circulatory system begins 

 in the roots, follows the trunk, 

 branches, stems and petioles, and 

 spreads out in a finely ramified net- 

 work of veins in the leaf, the latter 

 being merely the extension of the 

 fibro-vascular bundles from the peti- 

 ole into the leaf. 



Circulation in the plant is depen- 

 dent upon evaporation from its 

 surfaces. The leaves promote evap- 

 oration, and the lost moisture is 

 made good mainly by absorption of 

 water through the roots. There are 

 reasons for supposing that the sto- 

 mata are the chief organs of evap- 

 oration, especially in those plants 

 which have a dense and imper- 

 vious cuticle. But there are also 

 good reasons for believing that the 

 stomata have other offices besides 



merely facilitating evaporation. There 

 can be but little doubt that they are 

 also the means of communication be- 

 tween the external atmosphere and 

 the internal chlorophyll-bearing cells 

 through which the supply of materials 

 is obtained for the process of assim- 

 ilation already described. Such a 

 relation between the stomata and the 

 chlorophyll-cells is at once suggested 

 by the fact that the stomata are invari- 

 ably directly over intercellular cavi- 

 ties ; and it is confirmed by the fact 

 that they open in the presence of light 

 and close in darkness, being ready 

 for duty only at a time when the 

 assimilative process is active. If the 

 function of evaporation were their 

 sole office, one would suppose that 

 they would be most open when evap- 

 oration was least active (in the shade), 

 and more closed when evaporation 

 was likely to be too rapid (in the sun- 

 light), acting, in this respect, like the 

 governor on an engine. 



While there is much evidence that 

 the stomata are organs of the assimila- 

 tive process, there is quite as much 

 evidence that they are not connected 

 with the metastatic process. Of this 

 negative evidence we recall at once 

 the fact that the stomata are never 

 found upon the fibro-vascular veins of 

 the leaf. Other negative evidence 

 will suggest itself as the converse of 

 postive evidence which will hereafter 

 be mentioned as showing that the 

 hairs are connected with, and are 

 organs of, the metastatic process. 



Several recent observers of plant- 

 hairs have attempted to account 

 for their existence ; but no one, as 

 far as I know, has done so upon 

 purely physiological grounds. To 

 my mind, however, the evidence that 

 hairs are primarily organs of the 

 metastatic process, strengthens with 

 each new fact that comes to my know- 

 ledge. This theory is not affected 

 by the fact that not all plants 

 in all stages of their existence 

 are hairy, because, probably no pro- 

 cess in vegetable physiology is per- 

 formed in precisely the same manner, 



