16 MORPHOLOGY 



Structure. The structure of the protoplast oiChlamydomonas(&%. 21) 

 may be taken as representative of the whole group. There is usually a 

 single, large, cup-shaped chloroplast at the larger end of the protoplast, 

 in which is embedded a large protein body (pyrenoid). In the cup of 

 the chloroplast the nucleus is found; near the base of the cilia are two 

 contractile vacuoles*, at the forward end a red pigment spot ("eye spot") 

 is observed ; and two long apical cilia complete the equipment. The 

 cells are very active and their motion is influenced by light, to which the 

 "eye spot" is supposed (without adequate ground) to be very sensitive. 

 In some forms a red pigment appears so abundantly as to give a reddish 

 hue, giving rise to the accounts of " red pools" and "red snow." Under 

 certain conditions the cell may drop its cilia and become quiescent, and 

 this temporary loss of motility in the vegetative cells of Volvocales 

 becomes the permanent condition in higher forms. 



Reproduction. In this quiescent stage, the protoplast may divide 

 into several new cells, which escape as new and active individuals. 

 These daughter cells, formed within the old mother cell, are called 

 zoos pores (swimming spores, swarm spores), but they are also the adult 

 form of the plant. Therefore, the ordinary vegetative cells of Volvo- 

 cales are like the zoospores of the higher forms, in which the vegetative 

 cells and zoospores are quite distinct. 



Certain cells form more numerous and smaller zoospore-like cells, 

 which escape, swim freely, and fuse in pairs to form new cells (figs. 24- 

 27). This is a sexual process, and therefore these pairing cells are 

 called gametes (sexual cells). Since the pairing gametes are alike, the 

 condition is called isogamy, and the plants are said to be isogamous. It 

 is evident that the gametes are related to the zoospores, and it is thought 

 that they are only modified zoospores. The origin of gametes is the 

 origin of sex, and isogamy is the simplest form of sexuality. This fusion 

 of two gametes to form a single cell is the act of fertilization, but to 

 distinguish it from the higher forms of fertilization it has become cus- 

 tomary to call the fusion of similar gametes conjugation. The cell 

 resulting from conjugation is a zygospore (or zygote), spore being the 

 general name of a cell set apart for reproduction, and the prefix in this 

 case indicating that the spore has been formed by conjugation. In 

 isogamous plants, therefore, the gametes conjugate and produce a 

 zygospore. In general the zygospore is a resting cell, being formed at 

 the inception of unfavorable conditions, having a heavy wall, and 

 starting new generations upon the return of favorable conditions. In 



