Evidence from Somatic Histogenesis 57 



characters of plants that are referable to individual cells I 

 do not know; but judging from the instances that come to 

 view even in my limited knowledge of the subject it might 

 be carried to an almost indefinite extent. Another instance 

 which I recall from the experience of my student days is 

 the case of mosses. The serration of the leaves, I remem- 

 ber, was one of the features relied upon for generic and 

 specific characteristics, and I also remember that the indi- 

 vidual teeth often if not always consisted of one or a very 

 few cells. 



Cell-wall Structures In Higher Plants 



That the cell-wall is a structure of great importance in 

 plants is known to everybody; and the veriest tyro in plant 

 histology has learned something of the enormous variety 

 and definiteness of character in different tissues and dif- 

 ferent kinds of plants presented by this part of the cell. 



A very brief reference to two plant structures, pollen- 

 grains and wood tissue, will be, perhaps, a sufficient re- 

 minder of what there is for us in this domain. A typically 

 formed pollen-grain is a minute spheroidal body containing 

 two cells, one known as the antheridial or germinative cell 

 and the other as the sterile or vegetative cell. The wall of 

 the grain consists of an outer coat, the exine, and an inner, 

 the intine. The elaborateness of structure which these coats 

 may reach is astonishing if regarded in the crepuscular light 

 of the theory that cells are "simple" things. The most dis- 

 tinctive thing about the pollen-grain is the pollen tube 

 which is produced on one side of the grain and through 

 which the antheridial cell reaches the ovule in fertilization. 

 The taxonomic variety which is our main interest just here 

 pertains largely to the sculpturing of the surface of the 

 exine and to the structure of the exine at the point where 

 the pollen tube will break through. It is well known to 

 botanists that the "spikes, warts, ridges, combs, etc." of 



