488 SECOADARF CHAXGES. 



importance is attached to their contents. The intermediate fibres are separated from 

 the fibrous cells, also on account of the structure of their walls, and placed, with the 

 parenchyma of the bundles, in Category I. The rest of the classification resembles our 

 own. In so far as nothing further is aimed at than an intelligible arrangement of the 

 forms of tissue of the secondary wood, Sanio's classification is without doubt as intelli- 

 gible as our own, and perhaps more so. Both also suffer from the same defect, namely, 

 that the categories distinguished cannot always be sharply separated, and in particular 

 that intermediate forms occur between fibres and tracheides, &c., as has often been 

 stated above. Both, however, afford guidance in each particular case, in accordance with 

 the system adopted. There would therefore be no reason for undertaking alterations in 

 Sanio's arrangement, if it were not an essential object to refer the forms of tissue in the 

 secondary wood to their proper place among those distinguished in the plant generally, 

 including those outside the secondary wood. It can admit of no doubt that the elements 

 of the secondary wood are not organs sui generis, but belong to the forms of tissue which 

 have been characterised in this bonk as tracheae, sclerenchymatous fibres, and cells ; the 

 last being distinguished from the others by permanently containing protoplasm, or in 

 doubtful cases by their periodically varying store of starch (comp. pp. 5 and 115). The 

 known phenomena of the secondary wood present, as I believe, no argument against the 

 general classification of the forms of tissue carried out in this book, for the occurrence of 

 intermediate phenomena cannot avail as an argument against the distinction of typical 

 forms. In the face of these facts it was, however, necessary to depart in some points 

 from Sanio's classification, which was based on other considerations. 



I willingly grant that the sharp separation of the cells from the other elements is often 

 inconvenient for the practical description or identification of woods, as it is not always 

 an easy matter to establish the cellular quality. In most cases indeed the presence of 

 starch is a certain character, both in the fresh and in the dry wood. In its absence, 

 however, the distinction is in the latter often impossible or only possible with great diflfi- 

 . culty. In the wood of Cobaea, for example, which has been preserved dry, it can scarcely 

 be determined with certainty whether the numerous reticulated elements with large pits 

 are short tracheides or parenchymatous cells, for in the structure of their walls they 

 resemble reticulated tracheae, which might occur, and all the formed contents have 

 become indistinguishable. In the fresh plant, on the other hand, the presence of proto- 

 plasm and with it the cellular quality may be recognised, at least up to the third year, 

 by means of the large chlorophyll grains. Experiences of this kind are, as we have said, 

 inconvenient, though, on the other hand, they are certainly instructive, as they point to 

 the necessity of investigating woods in the fresh living condition, more than is usually 

 done. They would, however, only present a serious objection to the classification arrived 

 at, it the problem of the anatomy of wood were to be sought in the construction of a 

 convenient 'key' for description and identification. 



3. Distribution of the tissues in the wood. 



Skct. 146. The distribution of tissues in the wood, and the consequent structure 

 of the latter, is uniform in the successive annual rings, with the exception of certain 

 differences to be specially treated of below ; the single annual ring may therefore be 

 considered first. In the few cases without annual rings the description applies to 

 the entire woody ring. 



As has already been often mentioned, alternate radial bands of unlike structure 

 are nearly always apparent at the first glance: (i) the medullary rays, and (2) the 

 ligneous bundles. This fact determines the main subdivisions of the exposition. 

 Where this alternation of unlike radial bands does not exist, as in the cases men- 



