58 The Unity of the Organism 



the surface of the grains are in general definitive of taxo- 

 nomic groups of plants. And concerning the places of 

 emergence of the pollen tube we read: "The number of 

 these peculiarly organized points of exit is a fixed one in 

 each species, and often in whole genera and families." 

 As to the way these various structures are produced we have 

 this very definite statement: "The sculpturing upon the 

 outer surfaces of the spores of mosses and ferns and the 

 corresponding pollen grains of the Phanerogams can in 

 most cases be attributed to the activity of the protoplasm 

 surrounding the developing spores." 19 



That the main tissues of plants present taxonomic char- 

 acters is amply illustrated in the wood-tissues. The facts 

 concerning these tissues have been almost forced into 

 prominence by the needs of fossil botany, though they have 

 also been much studied as a part of ordinary plant mor- 

 phology. The section on fossil wood in Zittel's Handbook of 

 Palaeontology is a considerable resume of knowledge in this 

 field, and contains numerous statements and figures which 

 bring out impressively the general truth of the specificity 

 of the tissues of trees. After necessary allowance is made 

 for the strictly botanical unsatisfactoriness of many of the 

 species and genera recognized by palaeobotanists, it is not 

 doubted, so far as I know, that on the whole the kinds of 

 tissue they describe do in reality represent different kinds 

 of trees. A single illustrative quotation will serve to make 

 concrete what is here dealt with in general terms : "The 

 phloem segments, like those of the xylem, are divided by 

 few-seriate pith rays into rather regular two- to four- 

 seriate rows of cells made up of thin-walled, small-celled 

 elements in the trunk of Zamia ftoridana, etc., and Stan- 

 geria. But ... in the trunks of Cycas, Dion, Encepha- 

 lartos, Macrozamia and doubtless most cycads, as likewise 

 in the Cycadeoideae, sclerenchymatous elements are more 

 or less numerously and regularly interspersed among the 



