August, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



in the second, to the various external forces that are 

 brought to bear upon it. The fact that in some cells, 

 notably those of hairs from the stamina of Tradescantia, 

 the protoplasm has the power of movement, that is, is 

 carried round the cell in varying directions, and that 

 this movement dependsvupon a number of conditions, 

 such as temperature, a requisite supply of oxygen, :ind 

 some others, shows that this substance is " irritable," 

 or, in other vi^ords, has the property of responding lo 

 stimuli. 



An interesting fact is the observation that root-hairs, 

 which are very elongated, delicate cells, can exert a 

 certain power of selection when surrounded by a solution 

 of various salts (a nutrient solution so-called) and this 

 shows that the peripheral protoplasm of these cells is 

 more irritable to certain forms of chemical stimuli than 

 to others. Likewise the experiments upon masses of 

 naked protoplasm, although here we are more rightly 

 considering the cells of higher plants, show that it is 

 directly irritable to other forms of energy, such as light 

 and electricity. These experiments are to- us useful, 

 from the point of view of comparison, for, all things 

 considered, the main difference is that in the case of 

 cells of higher plants, which we are considering, a cell- 

 wall exists, which in some way m.odifies the action of 

 the stimuli. 



We have called attention a little further back to 

 certain intercellular communicating fibrils of proto- 

 plasm. These fibrils have been thought to be of use 

 in the conduction of stimuli from any given cell to 

 adjacent cells, and so on to those more remote. There 

 is some evidence to show that this communication by 

 fibrils is of value to a plant much in the way that so~ 

 called ' ' trophic nerves ' ' are to an animal with a well- 

 marked nervous system, for cells that have been 

 partially injured may revive if still in communication 

 with other cells. But more proof is required before 

 this can be definitely settled. One thing is, however, 

 certain, namely, that nowadays one would no more 

 think of calling a living cell an isolated unit than say 

 that the cortical cells of the brain are functionally 

 isolated portions of the central nervous system. 



We must not, however, look upon the main mass 

 of protoplasm in a cell as the only irritable part; the 

 nucleus, the plastids, and, at certain times also, the 

 ccntrospheres are influenced by stimuli reaching them 

 either directly or from adjacent cells, for, as we have 

 seen, these structures are one and all specialised por- 

 tions of protoplasm. The plastids, however, arc 

 mainly irritable to certain chemical stimuli, produced 

 chiefly by carbohydrates in solution, and, in the case 

 of the chloroplasts, the pigment chlorophyll sifts out 

 certain portions of the radiant energy reaching them 

 from the outside and, co-operating with the chemical 

 stimulus, so acts upon the protoplasm of the plastid 

 that this latter is enabled to form starch. There is 

 evidence for believing that the actual protoplasm of the 

 chloroplast becomes transformed into starch, and, 

 moreover, it has been shown that protcids can, as a 

 rule, be split up into nitrogenous and carbohydrate 

 portions, so that the transformation in the plastid is 

 not so astonishing as at first sight appears. The same 

 tiling probably occurs during the formation of the oell- 

 plate in indirect or mitotic cell-division. 



The protoplasm responds to iiutrition by (a) growth 

 of its substance, and {l>) growth of the whole cell. 

 Assimilation is, howe\er, the result of a number of 

 stimuli, some chemical, produced by the various food- 

 materials, others physical and de[X!nding upon light and 



temperature. It is the combination of these and a cer- 

 tain unexplainable (as yet) vital activity* which 

 constitute metabolism, a term which includes both the 

 building up and breaking down processes essential to 

 the life of the protoplasm. 



There are certain factors in the response of proto- 

 plasm to stimuli which must be present in order that 

 -.uch a response may be called out. These are, the 

 presence of oxygen and a good supply of water, in 

 which latter the oxygen is usually held in solution to a 

 small extent. The existence of free oxygen in a cell 

 enables the katabolic side of metabolism to be carried 

 on, and the process is certainly as essential to the life 

 )f the cell in plants as it is to the animal cell. Water, 

 on the other hand, is equally essential, for the proto- 

 plasm is largely composed of water, both chemically 

 ;md physically speaking, and in the transport of 

 materials from cell to eel) it is of the highest impor- 

 tance. 



These two substances, then, oxygen and water, being 

 present, the protoplasm is in a condition to utilise and 

 react to the various forms of energy which reach the 

 cell from the outside, and the result is that the 

 organism is enabled to carry on a number of processes, 

 both physical and chemical, and in so doing, increase 

 in volume, change in shape, and the production of other 

 cells are brought about. That protoplasm has a direc- 

 tive action is evident from the fact that certain organs 

 grow in different directions, but after all, this directive 

 action is the result of the reaction of protoplasm to 

 certain external stimuli, such as gravity (geotropism) 

 of moisture (hydrotopism). The action of gravity in 

 causing a root-tip to bend downwards is well estab- 

 lished by experiment, and the discovery that a force in 

 any direction (centripetal force) could replace gravity, 

 has shown that it is really the given for;e which has 

 influenced the directive action of the protoplasm of the 

 individual cells. The true nature of this action is as 

 yet obscure, but there are grounds for assuming that 

 an attractive force, such as those we have taken as in- 

 stances, in some way modifies the intensity of 

 metabolism so as to^ cause greater rapidity of growth 

 in those portions of a cell which are furthest from 

 the attraction. 



The fact that certain flowers will turn towards or 

 away from light is another well-known instance of the 

 reaction of protoplasm to external stimuli, and on the 

 whole is due to the same cause, namely, increased 

 growth on that side which is turned away from the 

 stimulus. It is a well ascertained fact that more rapid 

 gnjwlh takes place in darkness, when transpiration is 

 at a minimum, than in the light, and the phenomenon 

 of heliotropism is thus not astonishing, as those por- 

 tions of a plant which are directed towards the most 

 intense illumination should transpire most, and grow 

 least rapidly, whereas in those parts turned away from 

 the light the reverse should be the case. But although 

 these observations serve as a gross explanation, they 

 do not show us the true reason for the increased activity 

 v)f the protoplasm. It is true that transpiring cells are 

 in general less turgid than non-transpiring ones, and 

 turgiditv favours growth, so that here we may have 

 a partial explanation. 



Turning now to that point to which we directed at- 

 tention some little way back, namely, that a living 

 cell must not only be looked upon as an individual 



• There appears to be some doubt as to the legitimacy of the use 

 of the term " vital-activity." See Huxley on " The Physical 

 Basis of Life," in " Method and Results." 



