ZOOLOGY AND BOTANY, MIOROSOOPY, ETC. 253 



orders. The uniformity of the results in particular genera of widely 

 separated orders shows that the diiferences in question are the result 

 of external conditions rather than of hereditary tendencies ; the fol- 

 lowing are the more important points in which the tissues become 

 modified by being buried in the soil. 



The epidermis, when present, is modified. Suberin attacks first of 

 all its external wall, and may even form a very thick layer ; it ascends 

 only slowly into the lateral and internal walls. The cortex increases, 

 either by increase of the size or number of its cells. The collen- 

 chyma either diminishes or disappears altogether, especially when 

 this tissue is enveloped in the angles of the aerial stem. There is a 

 tendency towards the early production of a suberous layer, which 

 appears at diiferent points of the epidermis, in the cortical paren- 

 chyma, in the endoderm, in the peripheral layer, and in the liber. 

 This layer is sometimes a substitute for a ring of fibres which is often 

 found outside the liber-bundles in the aerial stem. The under- 

 ground stem sometimes contains a few fibres, but they are much less 

 numerous. 



In the greater number of perennial plants examined the liber- 

 bundles of the aerial stem are closed, being shut up in this ring of 

 fibres ; while in the underground stem they are open. The activity 

 of the formative layer is very variable ; but lignification almost 

 always takes place irregularly in the woody bundles. The pith is 

 less developed in proportion to the cortex than in the aerial parts. 

 Food-materials, especially starch, exist in it in great abundance. 

 The angles of the aerial stem, when projecting, tend to disappear. 



The following phenomena in the underground stem may therefore 

 be attributed to the influence of the environment : — The great deve- 

 lopment of protective tissues, such as a suberous layer and a suberized 

 epidermis ; the reduction or disappearance of the means of support, 

 collenchyma, liber-fibres, &c. ; the great development of cortex and 

 relative reduction of pith ; feeble lignification ; and the production of 

 reserve food-materials. 



The proportion of perennial plants increases with the altitude 

 above the sea-level ; and the same species is sometimes annual at low 

 altitudes, perennial at high altitudes. The duration of a plant, there- 

 fore, and the presence of a rhizome or other form of underground stem, 

 are to a certain extent dependent on external circumstances. 



Junction of Root and Stem in Dicotyledons and Monocoty- 

 ledons.* — M. C. Potter draws the following comparison between the 

 passage from root to stem in these two classes of plants: — In the 

 procambium of the root the protoxylem or spiral vessels and the 

 protophloem or bast-fibres are first differentiated, the differentiation in 

 each bundle proceeding from without inwards, and thus the separate 

 xylem and phloem bundles are produced. In the stem each bundle 

 consists of xylem and phloem. The protoxylem is first differentiated 

 at the most external part of each bundle, and the differentiation pro- 

 ceeds from within outwards, while the protophloem is first differentiated 



* Proc. Camb. Phil. Sue, iv. (1883) pp. S'JS-d (1 pi.). 



