266 Mr G. F. Scott Elliot on the 



teeth in the upper surface, which press against each other 

 and so protect the more tender tissues. 

 III. The Nutritive Tissues — 



(a) Absorption System.— K&^eQ,\Q\ absorption system may 

 be absent, e.g., Algae. In aquatic plants, and plants with a 

 low transpiring power, e.g., Conifers and Psilotum, a root 

 epithelium is found,but no root hairs. These root hairs have a 

 special function in absorption. It appears most probable, that 

 the nutritive material required for vegetation exists in the 

 form of insoluble salts lying in water in the particles of earth. 

 Now, when a root hair meets one of these particles of earth, 

 it spreads itself out over it, or even curls round it. The 

 membrane of the root hair would be thus in connection with 

 the little drop of water, and the carbonic acid partly due 

 to respiration from the root hair would dissolve the 

 salts. There are many special absorption tissues, e.g., spongy 

 aerial roots of certain tropical orchids. Also, the cells of 

 many mosses, e.g., Sphagnaceae, mycelia of fungi, and similar 

 mycelia in many parasites {Thesium pratense), and sapro- 

 phytes. A very similar structure is found in the young 

 plant while nourished from the parent. 



(b) Assimilation System consists of those cells whose 

 primary function it is to produce organic matter from carbonic 

 acid and water. The chlorophyll grains are usually of very 

 small size in the higher plants, probably to aflford a larger 

 surface compared with their size. They are usually arranged 

 round the sides of the cell, perhaps to allow as much light 

 as possible to pass to the cells below. The palisade cells 

 are usually parallel to the direction of incident light, being 

 elongated in that direction, and this explains why in vertical 

 leaves they are sometimes found pointing obliquely upwards. 

 They may be formed on the lower surface when this is 

 illuminated. A great chemical advantage is gained if the 

 already manufactured products are conducted away as 

 quickly as possible, and this principle seems to have a great 

 influence on the arrangement of the assimilation system. 



Three types may be distinguished — 



First, Assimilation and conducting tissues in one, as in 

 Mos.-es. 



Secondly, A single conducting system. This includes 

 most Monocotyledons, Pines, and other Conifers. 



