FORMS AND SYSTEMS OF TISSUES. 



9^ 



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 glands 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 lithocysts 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 examiples 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'h 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 walls 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 starch particles may be made to pass through the 

 perforated partition from one cell to another.] 



^ Sachs, Experimental- Physiologic, p. 386. 



3 [See also Meyen, Ueber die Secretionsorgane der Pflanzen, Berlin 1837.— J. B. Martinet, 

 Organes de secretion des vegetaux : Ann. des Sci. 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 somewhat more restricted sense tTian is usual in English botanical 

 works, in consequence of the etymological meaning of the corresponding German term * Driise,* 

 in which the idea of something compound is implied.] 



' See Sect. 15, under the head of Epidermis, p. loi. 



