45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 203 



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, nitration 

 under pressure 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 communicating with the 

 surface by two or three water-stotnata (see p. 109). 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 holding 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. 



In connexion with the catabolic processes there is an evolution 

 of energy constantly going on in the plant, which is for the most 

 part lost to the plant, or dissipated, most commonly in the form of 

 heat, in a few cases in the form of light, and also commonly in the 

 form of movement. The evolution of heat by plants is not usually 

 sufficient to cause the temperature of the plant-body to be higher 

 than that of the surrounding air. This is partly due to the fact 

 that the catabolic processes of plants are not generally very active, 

 and partly to the continual loss of heat by radiation and in con- 

 nexion with transpiration. It is however easy, under appropriate 

 conditions, to demonstrate the evolution of heat. If a quantity of 

 seeds be made to germinate in a heap, they will be found to be 

 distinctly warm (Fig. 129). This happens on a large scale in the 

 process of malting barley. When a large quantity of barley -grains 

 are germinating on a malting-floor, they become quite hot : they 

 have, in fact, to be continually turned to prevent overheating. 

 The conditions are here most favourable : for the catabolic pro- 

 cesses are extremely active in germinating seeds, and there is but 

 little loss of heat by radiation and transpiration. Similar observa- 

 tions may be made with opening flower-buds, the opening of the 

 bud being also a period of great catabolic activity. In some cases, 



