CHAPTER V. 



OF THE STRUCTURE AND FUNCTIONS OF THE STEM. 



123. THE chief office of the Stem appears to be, to elevate 

 the leaves which are organs destined to convert the crude fluid 

 absorbed by the roots, into nutritious sap for the supply of food 

 to the structure, and the flowers, in which a part of this sap 

 is applied to the production of new individuals, into the most 

 favourable position for receiving the influence of light, heat, and 

 air, on which their due actions depend. Accordingly we usually 

 find it most developed, in those kinds of plants, in which one 

 portion of the surface is set apart for the absorption of fluid, and 

 another for its exposure to these influences. In the little plant 

 which constitutes the Red Snow, and in others of a similar grade 

 of organisation, we find the whole surface adapted to absorb, and 

 the whole surface equally exposed to air ; there is, therefore, no 

 necessity for a stem. But as soon as, in the Mushroom tribe for 

 example, the plant sends roots into the earth, it elevates the 

 other portion above it by means of a stem ; and in this stem 

 there is a set of channels or passages, which serve to convey the 

 absorbed fluid from below upwards. But in these humble plants, 

 destined to live but for a short time, and then speedily to decay, 

 there is no necessity for providing the short stem with the tough- 

 ness required in the trunk of the lofty forest-tree, which braves 

 the storms of centuries. And accordingly we find that, whilst 

 in the former the tissue is soft and cellular, resembling that of 

 the rest of the structure, in the latter it is firm, consisting almost 

 entirely of woody fibre ; and that this is consolidated, by the 

 deposition of hard matter within its tubes. 



124. Between these two extremes of softness and toughness, 



