124 



PLANT ANATOMY. 



(1) The mucilaginous modification is 

 seen in Chondrus crispus, and in many 

 seeds, as in Quince, Flax, Mustard, Cas- 

 sia fistula, etc. Such walls have the 

 property of swelling to a great extent 

 upon the addition of water, and all de- 

 grees may be traced, from pure cellu- 

 lose, which is insoluble in water, to pure 

 gum or mucilage, which may be entirely 

 soluble. 



(2) Lignification is supposed to be due 

 to an incrustation of Lignin or woody 

 substance upon the cellulose of the cell 

 wall. This lignin is a hypothetical 

 substance which gives a number of cer- 

 tain micro-chemical reactions, but its na- 

 ture is far from being understood. It 

 gives to cell walls a toughness and elas- 

 ticity that are characteristic of wood 

 and bast fibres. 



(3) Suberization is seen in cells of 

 cork; it renders them elastic and imper- 

 meable to water. It has been taught 

 that it is due to a deposit of fat-like sub- 

 stance upon the cellulose of the wall; 

 but many investigators have claimed 

 that there is no cellulose in cork cell 

 walls, and that suberin is a separate 

 chemical substance. 



(4) Cutinization is the process that 

 takes place in the epidermal tissues of 

 the plant, generally in the leaves and the 

 outside covering of fruits. The proper- 

 ties of Cutin are similar to those of Su- 

 berin. Micro-chemically they agree. 



(5) Mineralization. Mineral salts, par-. 



ticularly carbonates and silicates, are 

 sometimes deposited in the walls of cer- 

 tain plant cells, as in grasses, horse 

 tails, etc. Their function seems to be 

 purely mechanical. 



CHAPTER V. 



THE GROWTH OF CELLS TO FORM TISSUES. 



If the evolutionary point of view be 

 kept in mind, it becomes necessary to 

 trace the growth of highly-complicated 

 tissues, as found in the higher plants, by 

 means of the simpler forms of the lower 

 plants (Phylogeny), or through the in- 

 dividual stages of growth from the fer- 

 tized ovule to the fully developed plant 

 (Ontogeny). Owing to the gaps in 

 Paleobotany the Phylogenetic relation- 

 ships of plants are difficult to trace, and 

 the study of plant embryology affords a 

 simpler clue to the origin of complex 

 structures. 



CET^L DIVISION. The two main 

 types of cell division have been de- 

 scribed, the direct and the indirect. 



1. The Direct method consists of the 

 splitting of a cell into two, wMthout any 

 nuclear mechanism- It 

 mainly to the lower forms 

 life. 



2. The Indirect method (Mitosis, or 

 Karyokinesis), afEords an interesting and 

 complicated study, which is mainly cen- 

 tered in the nucleus. It hn>: been pointed 

 out that the nucleus consists of several 

 protoplasmic substances, the most im- 

 portant of which waa the Chromatin. 

 This is generally scattered about the 

 nucleus in the form of minute granules, 

 but when the process of karyokinesis is 

 about to take place it is noticed that the 

 chromatin commences to arrange itself 

 in irregular lines or a "skein." At the 



same time the Centrospheres, or **Attrac- 

 tion Spheres," two very minute bodies 

 at one side of the nucleus, outside of the 

 nuclear membrane, separate and wander 

 to opposite sides of the nucleus. These 

 spheres seem to exert some influence 

 upon the chromatin threads (Chromo- 

 somes), for they are soon found arranged 

 in the shape of a spindle reaching toward 

 the centrospheres, and are apparently ar- 

 ranged along radiating lines that stretch 

 from one attraction sphere to another. 

 (Achromatic Spindle.) 



IS confined 

 of vegetable 



Fig. n . - Epidermis cells of seed of Sinapis alba, 

 snowmgr mucilat^inous modification after treat- 

 with water. (Tschircli). 



F 



The second stage begins with the dis- 

 appearance of the nuclear membrane 

 and the longitudinal division of the 

 chromosomes. The chromosomes then 

 come closer together in the center and di- 

 vide transversely. (Metakinesis.) After 



