PHYSIOLOGY 247 



Selaginellas and Ferns, has nothing in common with actual phosphorescence, 

 but is produced solely by the reflection of the daylight from peculiarly formed 

 cells (Fig. 214). The phosphorescence observed in some seaweeds results, on the 

 other hand, from the fluorescence and opalescence of certain of their albuminous 

 substances, or from the iridescence of their cuticular layers ( 61 ). 



The light occasionally emitted by some floral structures on sultry nights is, we 

 must assume, electrical and comparable to St. Elmo's fire. 



IV. Growth 



The size which plants may attain varies enormously. A Mushroom 

 seems immeasurably large in contrast with a Micmcoccus, but small if 

 compared with a lofty Californian Sequoia. A Bacillus of the size of 

 a Mushroom, or a Mould-Fungus of the height of a Sequoia, are, 

 with their given organisation, physiologically as inconceivable as a 

 Mushroom with the minuteness of a Micrococcus. The size of an 

 organism accordingly is an expression of its distinct individuality, 

 and stands in the closest relation to structure and conditions of life, 

 and in the individuals of a species only varies within certain narrow 

 limits. 



However large a plant may be, and however numerous its cells, 

 it nevertheless began its existence as a single cell, microscopically 

 small and of the simplest structure. To attain its final size and 

 perfect development it must grow, that is, it must enlarge its body 

 and undergo differentiation. Even for the minute unicellular Bacteria 

 growth is essential, as they multiply chiefly by cell division. Each 

 daughter cell must grow and attain the dimensions of the parent cell, 

 or in a few generations the capacity for existence itself will be lost 

 through their continually decreasing size. It is in fact impossible 

 to conceive of a plant where development is not the result of 

 growth. If a growing Oak or Cedar be compared with the single 

 spherical egg-cell from which it has arisen, it is at once clear that by 

 the term growth we mean not only an increase in volume, but include 

 also a long series of various developmental stages, and external 

 and internal modifications. A mere increase in volume does not 

 necessarily imply growth, for no one would say that a dried and 

 shrivelled turnip grows when it swells in water. On the contrary, 

 active growth may be accompanied by a considerable loss of 

 substance, as is shown by the sprouting of potatoes kept in a dark 

 cellar ; water is lost through transpiration as well as organic 

 substance through respiration, and yet the new shoots show true 

 growth. 



In the lower organisms growth is exhibited in its most simple 

 form. In an amoeba or a plasmodium growth is simply an increase 

 in their substance ; in a unicellular Alga or in a Fungus it means, in 

 addition to this, an enlargement of their cell walls. In the higher 



