ABSORPTION AND ASCENT OF SAP. 27 



leaves (c). It is to be remarked also, that in general there is no 

 distinct bark, and that the external pellicle never grows in layers, 

 as is the case in the Ex'ogens. 



CELLULAR PLANTS never present parts that are really analo- 

 gous to the organs we have just spoken of, and to which we shall 

 again recur. 



LESSON III. 



MECHANISM OF THE ABSORPTION AND ASCENT OF THE SAP. 

 Ascending Sap Exhalation Respiration Leaves 

 Parts of Leaves ; their Structure, Shape, and Position 

 Stipules Tendrils Examples of the Forms of Simple and 

 Compound Leaves Exhalation Respiration Distribution 

 of the Nutritive Juices Descending Sap Secretions Ex- 

 cretions Succession of Crops Proper Juices Lignin 

 Fecula Growth of Plants Grafting Effects of the Sea- 

 sons on the Nutrition of Plants The Age of Plants. 



MECHANISM OF THE ABSORPTION AND ASCENT OF THE 



SAP. 



1. It is by the process of absorption that plants derive from 

 the soil in which they are fixed, the nutritive matters necessary 

 for their growth and the maintenance of their existence. 



2. The nutritive matters, to be pumped up in this manner, 

 must necessarily be in a fluid state ; in the solid form they could 

 not be absorbed ; and it is, in fact, water holding various sub- 

 stances in solution, that thus penetrates the plant and serves for 

 its nourishment. 



3. It is chiefly, and sometimes exclusively, by the extremity 

 of the roots that this operation is effected . The epidermis, 

 which covers almost the whole plant, in general offers obstacles 

 to the passage of these liquids ; but the spongioles, as we have 

 already seen, are unprovided with this envelope, and constitute 

 a cellular tissue which gives a ready passage to water ; for this 

 reason we must consider these spongioles as the chief organs of 

 absorption. 



4. Some plants also absorb by the leaves; and when the 



1. By what process do plants derive nourishment from the soil in which 

 they grow ? 



2. In what state or condition must the food of plants be before it can 

 nourish them ? 



3. What are the chief organs of absorption ? How is this operation 

 effected ? 



4 Is absorption carried on by the roots alone ? 



