q22 A TEXTBOOK OF THEORETICAL BOTANY 



A similar arrangement may be reached in quite a different way, as in 

 Amaranthiis, where the pericycHc secondary cambium remains permanently 

 active, and forms successive zones of ground tissue and of collateral vascular 

 bundles, all from its inner side, while on the outer side it forms only a little 

 phloem or nothing at all. 



9. Phloem Islands. This type of structure, also associated with many 

 climbers, is found in certain families of non-climbers, notably the Cheno- 

 podiaceae. The mature stem shows a vascular zone of normally arranged 

 xylem and phloem, but in the secondary xylem are a number of isolated 

 islands of phloem. These are formed by the enclosure of portions of the 

 normal phloem by short arcs of secondary cambium formed externally in the 

 pericycle. This cambium cuts off xylem on its inner side, that is to say, 

 outside the enclosed portion of phloem, which is thus bridged in by xylem 

 tissue (Fig. 906). 



The foregoing examples of anomalous structures are selected because they 

 have no relationship to any peculiarity in the growth habit or environment 

 of the plants, but are rather such as are characteristic of particular systematic 

 groups in which they are hereditary. They are thus " phyad " structures, 

 in the sense of Grisebach, rather than " ecads." The latter class of anomalies, 

 which are ecologically conditioned, will be dealt with in Volume IV. 



Morphological Modifications of the Stem 



The type of stem which may be regarded as normal is that which is 

 prevalent among Dicotyledons, namely an elongated, radially symmetrical 

 structure, bearing foliage leaves at nodes placed at graded intervals. Although 

 this corresponds to the general idea of a typical stem, it must be remembered 

 that throughout the Monocotyledons the stem is usually much contracted 

 and often subterranean, and that among the Dicotyledons there are large 

 groups within which the definition scarcely applies. We began this chapter 

 by emphasizing the protean nature of stems, and it needs only a superficial 

 acquaintance with plants to realize the truth of that remark. Even a wide 

 knowledge, if derived only from temperate floras, would give but a limited 

 idea of the morphological possibilities of the stem, and it would be rash 

 indeed to attempt to lay down general laws of stem structure or to formulate 

 a definition of a stem intended to be all-embracing. 



There is a considerable amount of " division of labour " among the shoots 

 of perennial plants, orthotropic shoots being often specialized for climbing 

 or for flower production, and plagiotropic shoots for various functions, 

 including especially food or water storage and the vegetative propagation 

 of the plant. These modifications are sometimes related to special environ- 

 ments or growth habits, and such will be dealt with in Volume IV. In the 

 majority of cases, however, they are not related to special conditions, but 

 are the result of, so to speak, inborn habit. How far such habitual modifica- 

 tions are fixed, i.e., genetical, and how far they are changeable by external 

 conditions, varies greatly in different cases. Few characters in plants are 



