730 PART IV. THE PHYSIOLOGY OF PLANTS. 



reference to two cases : to the nectaries, and to the chalk-glands. 

 The former afford an example of that mode of excretion in which 

 the necessary force is supplied by the excreting cells themselves : 

 the latter, of that mode in which the necessary force is derived 

 from another source. Excretion by nectaries can be well observed 

 in the case of Fritillaria imperialis (Fritillary, or Crown Imperial). 

 At the base of each of the petals of the flower, there is an 

 oval depression which is the gland or nectary and is seen to be 

 occupied by a large drop of nectar. If the flower be cut off, and 

 the drop be removed from the nectary by means of blotting-paper, 

 it will be shortly replaced by a fresh drop. It is therefore clear 

 that in this case the excretion of the liquid is effected, not by the 

 root-pressure, for the flower is no longer in connexion with the 

 root, but by the cells themselves. The mechanism of excretion 

 seems to be this, that the cells of the nectary become turgid, and 

 when a certain degree of turgidity has been attained, filtration 

 under pressure (see p. 669) takes place, and liquid is pressed out. 

 Excretion by chalk-glands can be well observed in some of the 

 Saxifrages. The chalk-glands are here situated at the end of the 

 finer vascular bundles round the margin of the leaves, each gland 

 being at the bottom of a depression in the surface, and communi- 

 cating with the surface by two or three water-stomata (see p. 157). 

 So long as the leaf is in connexion with the rest of the plant, and 

 provided that transpiration is not too active, drops of water hold- 

 ing chalk in solution are poured out by these glands on to the 

 surface through the water-stomata. The excretion stops, however, 

 directly the leaf is removed, or the stem is cut through. In this 

 case the excretion clearly depends upon the root-pressure; the 

 gland itself has no excreting power, but it simply accumulates 

 the chalk which is then washed out by the current of water 

 forced through the gland by the root-pressure. 



6. The Supply and the Expenditure of Energy. It has been 

 already insisted upon that a living organism must be supplied not 

 only with matter, but also with energy. The principal sources of 

 the supply of energy will now be briefly considered. 



a. All plants absorb kinetic energy in the form of heat (see 

 p. 671). 



6. Plants which possess chlorophyll absorb kinetic energy in the 

 form of light, the rays absorbed being those which correspond to 

 the absorption-bands of the chlorophyll-spectrum (see pp. 674 

 and 717). 



