44 FUNCTIONS OF BOOTS. 



in the Dahlia and in terrestrial Orchids. In epiphytic Orchids, 

 on the other hand, the roots are aerial, and the stems are much de- 

 veloped, forming pseudo-bulbs. Upon the roots of Spondias tube- 

 rosa there exist round black-coloured tubercules, about eight inches 

 in diameter, consisting internally of a white cellular substance, 

 which is full of water. These tubercules seem to be iatended to 

 supply water to the tree during the dry season. They are often dug 

 by travellers, each of them yielding about a pint of fluid of excellent 

 quality. 



Roots also give off excretions of different kinds. These are 

 eliminated by a process of exosmose (flow outwards), and con- 

 sist both of organic and inorganic matter. They were examined 

 by Macaire and Decandolle, and at one time they were thought 

 to be injurious to the plant, and by their accumulation to 

 cause its deterioration. It was also supposed that while they 

 were prejudicial to the species of plant which yielded them, they 

 were not so to others, and that hence a rotation of crops was neces- 

 sary. Daubeny and Gyde have found by experiment that these 

 excretions are not injurious, and it is now shown that the necessity 

 for rotation depends on the want of certain nutritive matters in the 

 son.* In very rich and fertile land the same crop may be grown 

 successively for many years. 



Stem or Ascending Axis. 



Forms of Stems, 



The stem is that part of a plant which bears the leaves and flowers. 

 It receives the name of OauUs in ordinary herbaceous plants which do 

 not form a woody stem, Culm in grasses, Truncus in trees, Gwudex or 

 Stock in Palms and in some Cacti, and Sti-pe in Ferns. Herba- 

 ceous stems are those of annual and biennial plants, as well as the 

 young yearly shoots of perennial plants. The term haulm is probably a 

 corruption of culm ; it is used by farmers to designate the stem of grasses 

 and the herbaceous stems of plants. The stem is not always conspicuous. 

 Plants with a distinct stem are called caulescent ;■ those in which it is 

 inconspicuous are acaules. Some plants are truly stemless, and con- 

 sist only of expansions of cellular tissue, called a Thallus, and hence 

 are denominated Thallogens, or Thallophytes (^aXXis, a frond, ymaeiv, 

 to produce, tpurhv, a plant). They have no true vascular system, but 

 are composed of cells of various sizes, which sometimes assume an 

 elongated tubular form, as in Ohara. The cells are sometimes united 



* This subject is considered when the sources whence plants derive their nourishment 

 are treated of. 



