34 FUNDAMENTALS OF CYTOLOGY 



of certain characters involving plastids suggests, however, that in some 

 but not all of these plants the male gamete contributes no functional 

 cytoplasm or plastids to the offspring, inheritance of such characters 

 being purely maternal in such cases. It seems probable that in ferns and 

 brj^ophytes, too, there is such a continuity of plastids. In these groups it 

 is plain that the motile male gamete loses most if not all of its cytoplasm 

 wdth any contained plastids before it unites with the egg. In Anthoceros, 

 mentioned in the preceding paragraph, the single plastid characterizes 

 all the cells in the life cycle with the exception of the male gamete. Again 

 in certain algae (Spirogyra, Zygnema) , whose gametes have essentially the 

 structure of vegetative cells, the plastids can be observed as distinct indi- 

 viduals through all the reproductive phases (Figs. 117, 118). 



In the absence of adequate evidence to the contrary, observations such 

 as the foregoing speak for the probable validity of the theory, long held by 

 many cytologists, that plastids are permanent protoplasmic organs always 

 derived from their predecessors by division. An element of doubt still 

 remains, however, for in certain instances it is reported that the proplastids 

 grade off to the lower limit of visibility, suggesting the possible presence 

 of newly formed ones as yet too small to be seen. Furthermore, it is 

 difficult to distinguish with certainty minute proplastids from other 

 equally small bodies in the cytoplasm. Origin anew might be suspected 

 in view of other cytoplasmic specializations during development. The 

 observation that in certain algal cells the chlorophyll is diffused through- 

 out the cytoplasm recalls the speculation that plastids as organs arose 

 historically when an important function performed throughout the cyto- 

 plasm became localized along with corresponding structural alterations in 

 the regions concerned. At present, however, there are not sufficient 

 grounds for stating that anything of this kind occurs in the development 

 of plastids by each individual plant. 



This section on plastids may be concluded with the reminder that it is 

 chloroplasts that make available to organisms the remarkable properties 

 of carbon with its capacity for forming the complex compounds required 

 as building materials and sources of energy. It is no wonder that a 

 famous botanist once said that he always felt like taking off his hat to the 

 chloroplast. 



Chondriosomes. — The presence of small bodies known as chondrio- 

 somes, or mitochondria, is all but universal in the cytoplasm of animals 

 and plants (Fig. 22). They appear in living cells as minute granules, 

 vesicles, rodlets, threads, and strings of beads, and they often vary in 

 abundance and form in different phases of cellular activity. Thej^ are 

 reported to arise anew in the cytoplasm, and they can be seen to divide; 

 in special cases only (e.g., certain animal spermatocytes) does the division 

 coincide with that of the cell. In many types of tissue they tend gradu- 



