LESSON 27.] CONVEYANCE OF THE SAP. 169 



tration is seen when we place powdered sugar upon strawberries, 

 and slightly moisten them : the dissolving sugar makes a solution 

 stronger than the juice in the cells of the fruit ; so this is gradually 

 drawn out. Also when pulpy fruits are boiled in a strong sirup ; as 

 soon as the sirup becomes denser than the juice in the fruit, the 

 latter begins to flow out and the fruit begins to shrivel. But when 

 shrivelled fruits are placed in weak sirup, or in water, they become 

 plump, because the flow then sets inwards, the juice in the cells being 

 denser than the water outside. Now the cells of the living plant 

 contain organic matter, in the form of mucilage, protoplasm, some- 

 times sugar, &c. ; and this particularly abounds in young and 

 growing parts, such as the tips of roots (Fig. 56), which, as is well 

 known, are the principal agents in absorbing moisture from the 

 ground. The contents of their cells being therefore always much 

 denser than the moisture outside (which is water containing a little 

 carbonic acid, &c., and a very minute quantity of earthy matter), 

 this moisture is constantly drawn into the root. What makes it 

 ascend to the leaves ? 



488. To answer this question, we must look to the leaves, and 

 consider what is going on there. For (however it may be in the 

 spring before the leaves are out), in a leafy plant or tree the sap is 

 not forced up from below, but is drawn up from above. Water large- 

 ly evaporates from the leaves (447) ; it flies off into the air as vapor, 

 leaving behind all the earthy and the organic matters, these not 

 being volatile ; the sap in the cells of the leaf therefore becomes 

 denser, and so draws upon the more watery contents of the cells of 

 the stalk, these upon those of the stem below, and so on, from cell to 

 cell down to the root, causing a flow from the roots to the leaves, 

 which begins in the latter, just as a wind begins in the direction 

 towards which it blows. Somewhat similarly, elaborated sap is 

 drawn into buds or any growing parts, where it is consolidated 

 into fabric, or is conveyed into tubers, roots, seeds, and the like, 

 in which it is condensed into starch and stored up for future use 

 (74, 103, &c.). 



489. So in absorbing moisture by the roots, and in conveying 

 the sap or the juices from cell to cell and from one part to another, 

 the plant appears to make use of a physical or inorganic force ; but 

 it manages and directs this as the purposes of the vegetable econ- 

 omy demand. Now, when the proper materials are brought to the 

 growing parts, growth takes place > and in growth the plant moves 



35 



