FORMS AND SYSTEMS OF TISSUES. 
91 
this transport to the more distant organs through the continuous tubes which they 
form. This is shown, among other evidence, by the course of these tubes, which is 
almost invariably in the direction of growth, and therefore enables them to place 
those organs in which food-material is produced in connection with those which 
require it. This is unquestionably the case with the sieve-tubes, which serve for 
the transport of the difficultly diffusible proteids, and secondarily also of the carbo- 
hydrates^. It is also true with respect to the laticiferous vessels, in so far as they 
contain proteids, oils, and carbohydrates. This function of the laticiferous vessels 
is not disproved by the fact'- that they usually also contain secretions that are not 
serviceable. The purpose, finally, of the wood-vessels is to form channels filled with 
air within the close woody tissue, replacing the air- conducting intercellular spaces of the 
succulent parenchyma. 
A totally different physiological function must, on the other hand, be assigned to the 
last form of coalescence of cells to be described, the Compound Glands. That these have 
no use connected with the transport of food-material is shown by their round form, 
which renders them quite unserviceable for placing different parts of the plant in com- 
munication. The same conclusion is indicated by the fact that the substances which 
accumulate in glanc's do not in any way contribute to growth, but must be regarded as 
excrementitious, or as secondary products of metabolism 
The popular usage of the term Gland is extremely indefinite, including not only 
single cells with peculiar contents, but also certain external organs like the nectaries 
of flowers and the colleters or glandular hairs of many leaf-buds. In this extended 
signification it is impossible to give an exact definition to the term. In order to 
get a good definition, we must exclude in the first place the bodies hitherto known 
as Unicellular Glands, which must be associated with hthocysts and gum-cells under the 
designation of Idioblasts, as the term has already been defined (p. 84). We may now 
define a Gland'^ as a group of cells sharply differentiated from those that surround 
them, whose intervening septa become absorbed, so that a single cavity is formed, 
which is often surrounded by special layers of tissue, and filled with excrementitious 
products, especially volatile oils. This definition excludes certain closely related forms 
of tissue, such as the nectaries and colleters"'"' already mentioned, which, however, in 
order to indicate their affinity, may be designated Gland-like bodies, in contradistinction 
to true glands. 
Good examples of glands in this sense are furnished by the large receptacles for 
volatile oil which occur abundantly in the rind of various species of Citrus. They 
may be recognised, even i^n the young ovary of the flower, as roundish groups of 
cells, distinguished by containing a turbid protoplasm and small drops of oil. The 
walls of these cells soon begin to swell, and the individual cells can be separated 
by pressure. The w^alls then deliquesce, and a large globular cavity is formed, 
^ See Sachs, Flora, 1863, p. 50. — Briosi, Bot. Zeit. 1873, nos. 20-22. [Briosi detected the 
presence of extremely finely-divided starch in the sieve-tubes of a large number of plants. Experi- 
ments lead him to think that by compression the slarch particles may be made to pass through the 
perforated partition from one cell to another.] 
^ Sachs, Experimental-Physiologic, p. 386. 
^ [See also Meyen, Ueber die Secretionsorgane der Pflanzen, Berlin 1837. — J. B, Martinet, 
Organes de secretion des vegetaux : Ann. des Sei. Nat. 5th ser. vol. XIV", 1872. It has been 
suggested that the contents of glands and similar secreting organs are not really excrementitious, but 
that they serve to protect the plant by preventing the consumption of the leaves &c. by insects 
and other animals.] 
* [The term is here used in a somevdiat more restricted sense than is usual in English botanical 
works, in consequence of the etymological meaning of the corresponding German term ' Drüse,' 
in which the idea of something compound is implied.] 
^ See Sect. 15, under the head of Epidermis, p. 101. 
