42 



CASSELL'S POPULAE GAEDENINa. 



accommodating — some much, more so than others — 

 and hence they are enabled to thrive under circum- 

 stances very different from those which are natural 

 to them. Again, while it is easy to imitate some of the 

 natural conditions, say temperature, it is often quite 

 out of our power to reproduce the attendant phe- 

 nomena of brilliant light or abimdant atmospheric 

 moisture. The home cultivator, while thankful for 

 any climatic indications with which the meteorolo- 

 gist and traveller can supply him, must study rather 

 to insure the particular combination and adjust- 

 ment of circimistances which will enable him to 

 grow the plant here. It is rarely possible to secure 

 the same combiuation of propitious circumstances 

 as exist ia the native country of the plant. For 

 these reasons we refer to the practical articles in 

 this volume, for indications as to the effect of tem- 

 perature on growth more directly useful than can be 

 obtained from a discussion of the-matter from the 

 point of view of the physiologist or meteorologist. 

 It may suffice to repeat in general terms what has 

 been said under the head of germination, that heat 

 and moisture are the two priacipal factors in pro- 

 moting growth. Light is less directly beneficial, 

 its most potent influence being exerted in other di- 

 rections, more especially in the nutritive processes. 

 It is obvious that, if growth is to be continued, some- 

 thing more than heat and moisture will be required, 

 because the reserve stores of food will sooner or later 

 be used up, and heat and moisture alone will not 

 suffice to build up new protoplasm or to accumulate 

 fresh stores of food. The moisture supplies only 

 two ingredients of the protoplasm (hydrogen and 

 oxygen) ; the heat itself supplies none, but simply 

 acts as a stimulant. It becomes necessary, there- 

 fore, to consider the food of plants, its nature and 

 sources, and this will involve the consideration of 

 the work done in the cells in general, and in those 

 of the root and leaf especially. 



The Work of tlie Cell.— The general structui-e 

 of the cell has been already alluded to. The work 

 it does may be brieflj^ summarised. As all plants 

 and all parts of plants are aggregates of cells, it 

 follows that the work done by the plant as a whole 

 is merely the sum of that done by individual cells. 

 The cell absorbs and exhales moisture and gases, 

 but it is incapable of absorbing solid matters, at 

 least no such material can pass the ceU-wall. It 

 changes and modifies the substances it absorbs ; 

 under the agency of heat and light it effects 

 changes at once more subtle and more vast than 

 the chemist has yet been able to bring about in his 

 laboratory; it contracts and expands; the old cell 

 divides and develops new ones. All this is effected 

 by the agency of the protoplasm. The ceU-wall does 



indeed act the part of a membrane in the purely 



physical process of osmosis, and it maj'- serve as a 

 shield and a protection ; but all the more important 

 work of the cell is done in and by the protoplasm. 

 Some of the cells remain unchanged, or at least 

 comparatively so, while others in course of growth 

 develop into fibres, vessels, and other elements of 

 plant structure which are told off, as it were, for 

 various offices, and correspondingly equipped. 



It is not consistent with our plan to go into detail 

 as to the form of cells ; but as we shall have frequent 

 occasion to mention them hereafter, we must allude 

 to one or two modifications. 



In the first place, we must again remind the reader 

 that the cell may exist without any membranous 

 cover at all, that at one stage of its existence a naked 

 and separate mass of protoplasm constitutes the 

 whole cell— the whole organism, in fact. Masses of 

 protoplasm of this character do not, as a rule, long 

 remain in this condition. They contract, or they 

 throw out long " feelers," as it were ; they be- 

 come, moreover, clothed with hair-like processes of 

 extreme tenuity,- caUed "cilia." By these means 

 they are enabled to move from place to place. At 

 other times masses of this character run together and 

 become incorporated into one mass, as invariably in 

 the reproductive process, and sooner or later each 

 one becomes clothed with a membranous covering, as 

 above mentioned. When this happens the ceU is 

 perfected, and its activity is in a measure arrested. 



Chlorophyll. — The first is the ceU with 

 " chlorophyll." Some cells contain no green colour- 

 ing matter at all, such are the cells constituting the 

 fungi at all periods of their life. Others, especially 

 those exposed to the light, contain in greater or less 

 proportion the green colouring matter called chloro- 

 phyll, the exact chemical nature of which is still a 

 matter of dispute, although no such difference of 

 opinion exists as to its extreme importance. We 

 shaU have to allude to it at greater length in con- 

 sidering the subject of leaf -action, and may, there- 

 fore, now confine ourselves to the general statement 

 that the special office of the cell containing chloro- 

 phyll is to absorb carbonic acid gas from the atmo- 

 sphere, to break up that combination of carbon and 

 oxvgen, to utilise the carbon, and set free the oxygen. 

 The cell without chlorophyll retains the oxygen and 

 gives out carbonic acid gas, as in the act of breathing. 



Wood-cells. — Another cell, to which we must 

 here allude, is the wood-cell. It is generally 

 elongated in form and thickened in the interior 

 with woody deposit. Aggregations of such cells go 

 to form the skeleton, as it were, of the plant, stiffen 

 it, and enable it to support itself, and to resist the 



