INTRODUCTION. 25 



of the water. Then we may refer to the almost identical form and structure of the 

 roots of the large majority of plants however different systematically, and to the 

 peculiarities, which at once appear in these, where a special adaptation takes place, as, 

 for instance, in the aerial roots of epiphytic orchids, the prop-roots of the Pandanacese, 

 Iriartese, &c. 



On the other hand, there are often to be found phenomena in the structure as well 

 as in the form of the vegetative organs, which may also be derived from adaptations, 

 which have happened in some epoch or other of the phylogenetic development, but 

 which cannot now be certainly referred to their causes ; properties, which were acquired 

 at an unknown time, and through unknown causes, are handed down to definite 

 series of successive generations, and at the present time are characters of Species, 

 Genera, Orders, and Classes, these corresponding to those taken from the formation 

 of flowers, embryos, &c. Of the more obvious phenomena of this category, we may 

 mention for example the arrangement of the vascular bundles in the stems and leaves 

 of most of the Monocotyledons and Dicotyledons, the structure of the vascular 

 bundles of the Ferns, of the wood in the Coniferae, and in most Chenopodiacese, &c. 



According to the terminology, which calls the properties, by which the divisions 

 of the system are characterised, its characters, we may term the (unexplained) pro- 

 perties of this category (unexplained) anatomical characters. 



Since the existing anatomical structure of a species is obviously the product of 

 the combination of the two categories of properties, it is to be expected beforehand 

 that,' as with external form, in different species, it will be more identical the nearer is 

 their affinity, and the more similar their adaptation. There are instances enough 

 where this is the case. The Coniferae, Filices, Chenopodiaceas, Cucurbitaceae may be 

 again cited on the one hand as groups which have from every point of view a similar 

 structure with very similar adaptation ; other groups, whose genera and species have 

 very different adaptation, show accordingly very different structural phenomena, for 

 instance, the Ranunculaceae (Ranunculus, Batrachium, Thalictrum, Clematis, &c.), 

 the Primulaceae (Lysimachia, Cyclamen, Hottonia, &c.) 



To this rule however any fairly extended investigation brings to light numerous 

 exceptions, viz. single species, genera, or groups, which, within their narrower or 

 broader circle of allies, which follow the rule, are characterised by definite peculiarities 

 of structure; these must, it is true, be regarded as inherited consequences of the 

 adaptations of the special ancestors of the plants in question, which however cannot 

 be referred to direct adaptation. Among the numerous cases, which belong to this 

 category, and which will be mentioned in the following chapters, we may cite as ex- 

 amples — the structure of the stem of the Auriculas (Primula auricula, &c.), which 

 differs so remarkably from that of the other Primulas, whose adaptations are however 

 not very different ; the wood of Strychnos, Wintera, &c. Examples of this sort show 

 how cautious one must be in adducing and. using anatomical characters for the 

 greater systematic groups ; how one must take care not to found such ideas upon 

 the structure of a couple of casually chosen species. 



The frequent occurrence of such exceptional cases makes the series of phe- 

 nomena, which are to be treated comparatively, highly complicated, and makes useless 

 the attempt, which at once suggests itself, to arrange the single sections, which treat 

 of the forms of tissue and their distribution, rigidly either according to the different 



