106 PLANT PHYSIOLOGY AND ECOLOGY 



a lichen thallus, and note the arrangement of the host-cells. Carefully 

 dissect the section in order to determine the connection between the 

 fungal filaments and the individual aglal cells. 



128. Growth. The growth of any plant is made possible 

 only by the growth of the individual cell, and, in multicellular 

 plants especially, by the division of the latter. The absorption 

 of water by a protoplast causes the stretching of the elastic cell 

 wall, and the accompanying assimilation of food results in an 

 increase in the amount of cytoplasm. The activity of the cyto- 

 plasm brings about a deposition of cellulose particles in the stretched 

 wall, and keeps it at its normal thickness. In a meristem cell 

 that has stopped dividing and has begun to stretch in conse- 

 quence of the absorption of water, the vacuoles of cell-sap in- 

 crease in bulk more rapidly than the cytoplasm. The nucleus, 

 moreover, shows a marked decrease in size, doubtless due to the 

 withdrawal of material from it as it loses the power of active 

 division. Thus, while the meristem cell contains a large nucleus 

 surrounded by much cytoplasm filled with many small vacuoles, 

 a single large vacuole is the most conspicuous feature of a paren- 

 chyma cell. The cytoplasm, while it has increased somewhat 

 in quantity, is now a thin layer closely applied to the inner sur- 

 face of the wall, and the nucleus has become more or less incon- 

 spicuous. In addition to the reinforcement of the cell wall by 

 the placing of new cellulose particles among the original ones, 

 the cytoplasm may add new layers of cellulose in those cells espe- 

 cially destined for mechanical support. These thickening layers 

 may be added almost uniformly, as in stone cells and fibers; at 

 the angles, as in most thick-angled tissue; or in various forms, 

 as in ringed, spiral, and reticulated vessels or tracheids. The 

 first layers usually consist of cellulose, but in the later ones this 

 is generally replaced by lignin in supportive tissues, and by cutin 

 in protective ones, as in the cuticle of the epidermis. The growth 

 of the cell becomes impossible after the wall is thickened or its 

 substance changed, since mechanical stretching is no longer possible. 



129. Growth of tissues and organs. The continued growth of a 

 mass, i.e., a tissue, is possible only when the individual cells in- 

 crease in number as well as in size. Since increase in number 

 is the regular consequence of the growth of the cell, the two always 

 occur together. Increase in the size of a cell is limited not only 

 by mechanical laws, but also, and especially, by the relation 



