78 DO SOLID SUBSTANCES ENTER THE ROOTS? 



of oxygen in the soil, it would further appear that those of some plants 

 require it more tlian those of others ; inasmuch as some genera, like the 

 grasses, love an open and friable soil, into which the air is more com- 

 pletely excluded. — [Sprengel, Chemie, II., p. 337.] 



III. We have in a former lecture (IV. p. 64) concluded from facts 

 there stated, that solid substances, which are soluble in water, accom- 

 pany this hquid when it enters into the circulation of the plant. This 

 appears to be true both of organic and inorganic substances. Potash, 

 soda, lime, and magnesia thus find their way into the interior of plants, 

 as well as those substances of animal and vegetable origin to which the 

 observations made in the fourth lecture were intended more especially to 

 apply. Even silica,* considered to be almost insoluble in water, enters 

 by the roots, and is found in some cases in considerable quantities in the 

 stem. Some persons have hence been led to conclude that solid sub- 

 stances, undissolved, if in a minute state of division, may be drawn into 

 the pores of the root and may then be carried by the sap upwards to the 

 stem. 



Considered as a mere question of vegetable mechanics, argued as such 

 among physiologists, it is of little moment whether we adopt or reject 

 this opinion. One phj'siologist may slate tliat the pores by which the 

 food enters into the roots are so minute as to baffle the powers of the best 

 constructed microscope, and, therefore, that to no particles of solid mat- 

 ter can they by possibility give admission — while another may believe 

 solid matter to be capable of a mechanical division eo minute as to pass 

 through the pores of the finest membrane. As to the mere fact itself, it 

 matters not which is right, or which of the two we follow. The adoption 

 of the latter opinion implies in itself merely that foreign substances, 

 unnecessary, perhaps injurious to vegetable life, may be carried forward 

 by the flowing juices until in some still part of the current, or in some 

 narrower vessel, they are arrested and there permanently lodged in the 

 solid substance of the plant. 



By inference, however, the adoption of this opinion implies also, that 

 the inorganic substances found in plants, — those which remain in the 

 form of ash when the plant is burned, — are accidental only, not essential 

 to its constitution. For since they may have been introduced in a mere 

 state of minute mechanical division suspended in the sap, they ought to 

 consist of such substances chiefly as the soil contains in the greatest 

 abundance, and they ought to vary in kind and relative quantity with 

 every variation in the soil. In a clay land the ash should consist chiefly 

 of alumina, f in a sandy soil chiefly of silica. But if, as chemical in- 

 quiry appears to indicate, the nature of the ash is not accidental, but es- 

 sential, and in some degree constant, even in very diflerent soils, this 

 latter inference is inadmissible; — and in reasoning backwards from this 

 fact, we find ourselves constrained to reject the opinion that substances 

 are capable of entering into the roots of plants in a solid state — and this 

 without reference at all to the mechanical question, as to the relative size 

 of the pores of the spongy roots or of the particles into which solid mat- 

 ter may be divided. 



* Silica is the name given by ctiemists to the pure matter of flint or of rock crystal. Sand 

 and sandstones consist almost entirely of silica, 

 t Alumina ia the pure earth of clay. 



