156 COMPENDIUM OF GENERAL BOTANY. 



ical differences in order to bring out the characteristics of an 

 organ. 



In botany an organ is a cell-portion, a cell, or a cell-complex 

 which is adapted to perform a definite life function of the plant. 



Morphological differences of organs do not always coincide with 

 physiological differences. Organs that are equal in importance 

 physiologically may differ very greatly morphologically, while 

 organs differing physiologically may have similar morphological 

 characters. Compare, for example, thorns and prickles, tendrils 

 and climbing sterns. Morphological definitions are dependent upon 

 the history of development ; physiological definitions upon func- 

 tion. Strictly speaking, morphology treats only of the members of 

 a plant-body, while physiology treats of the organs (Sachs). We 

 usually base the distinction of plant-organs upon morphological 

 differences, while the modification or formation of organs is based 

 upon physiology. The critical features of such a procedure will be 

 discussed below. 



1. The thallome. In the thallome there is no sharp differentia- 

 tion between stem and normal leaf ; this organ may resemble a stem 

 or a leaf, or it may resemble both organs in different parts of the 

 same plant. In its simplest form it is single-celled and not 

 branched (examples: Diatom acecK, Desmidiacece), or it may be 

 branched, consisting either of a single cell or of a few cells. The 

 lateral organs of the alga Scytonema are simply repetitions of the 

 mother-organ, while in other algse (Caulerpa, Fucacew, Floridece) 

 there is a marked distinction between the main plant-body and its 

 lateral organs; the latter may re present leafy formations or root-like 

 structures. The prothallium of ferns, a small green heart-shaped 

 structure found on the soil in the forest or in flower-pots, green- 

 houses, etc., is a thallome. A thallome is therefore an independent 

 vegetable structure devoid of organs, with perhaps the exception of 

 trichomes. 



The following organs are closely related to each other. The 

 thallome may occur independently, but the trichome, caulome, phyl- 

 lorne, and root cannot occur independently. 



2. The trichome originates from the superficial (epidermal] 

 cell-layer of various organs, or more rarely from the epidermis and 

 cells lying beneath the epidermis ; so that we may distinguish be- 

 tween epidermal trichomes and tissue-trichomes (emergences). The 

 spines on the fruit of the horse-chestnut, for example, are emer- 



