140 



COMPENDIUM OF GENERAL BOTANY. 



contact with water and particles of soil, to enable them to take up 

 and conduct food-substances in solution. Other substances not 

 soluble in water are rendered capable of being 

 taken up by the root-hairs. Some mineral sub- 

 stances are made soluble by a secretion of the root 

 itself, perhaps an organic acid. According to 

 SACHS, this may be demonstrated by means of a 

 polished marble plate, osteolith- or dolomite-plate, 

 upon which growing roots produce figures of 

 corrosion. Blue litmus paper is turned red by 

 this excretion of the roots. The activity of the 

 root-hairs also reduces or entirely removes certain 

 salts from the soil, as lime-salts, phosphates, and 

 compounds of ammonia. Besides the organic acid 

 referred to, roots also secrete CO a . 



In plants devoid of roots the soluble food-sub- 

 stances are taken up by the rhizoids, hair-like 

 structures met with among the prothallia of ferns, 

 and among the lichens and mosses. In Marchantia 

 these hair-like rhizoids possess peculiar elevated 

 80 - A A thickenings of the cell- wall which project inward ; 

 they have perhaps a mechanical function, namely, 



PIG. 



young seedling 



with particles 01 . /. ,-, 



soil adhering to to prevent collapse 01 tlie cells. 



the root-hairs. B, 



The same with 



soil- pa r tides 



washed away. 



(After Sachs, from 

 Frank.) 



(b) Aerial Roots. 



In the plants of moist warm climates a con- 

 dition artificially produced in our greenhouses 

 roots very frequently develop from aerial organs. Such roots 

 may subsequently enter the soil, in which case the subterranean 

 portion performs the function of an ordinary root; or they 

 may remain permanently suspended in the air, in which case 

 they are specially organized to serve as aerial organs (Aroideot, 

 epiphytic orchids). In the anatomy of true aerial roots there 

 is found just outside the normal root- cortex a covering of 

 several cell-layers in thickness called the velamen. The cells 

 of this layer are filled with air and the walls contain delicate 

 spiral or reticular thickenings. The special function of this cell- 

 layer is to absorb water-vapor. Between the velamen and the 

 cortex there is a layer of cells which is known as ' ' endoderm ; ' ' 



