GROUPS OF PLANTS. 7 



thread-like appendages known as cilia, which enable them to 

 move about in the water. They are known as zoospores or swarm 

 spores (Fig. 5, B, C), and each individual zoospore is able to 

 produce a new plant. 



The number of zoospores formed in a sporangium is usually 

 2 to 8, as in Ulothrix, but the number may be larger. The method 

 of cell formation which gives rise to zoospores is sometimes 

 spoken of as internal division from the fact that they arise 

 within the old cell and retain no relation to the old wall as is the 

 case in cell fission. The zoospores are at first naked protoplasts, 

 but later, on coming to rest, may form a wall. Sexual spores, on 

 the other hand, are formed by the union of two cells known as 

 GAMETES. When the gametes are similar the resulting spore is 

 known as a zygospore or zygote (Fig. ^, E, F, G). When the 

 gametes are unlike, the spore produced by their union is known 

 as an oosporeI In the latter case one of the gametes is larger 

 than the other, is less active, and is spoken of as the female 

 gamete, oosphere, or tgg (Figs. 11, 12). The other more active 

 cell is known as the male gamete, antherozoid or sperm (Fig. 

 34, ///). The cell giving rise to the oosphere is known as the 

 oogonium (Figs. 8, 11, 12), while the one in which the anthero- 

 zoid or sperm originates is called the antheridium (Figs. 8, 11, 

 12, 34). 



PLANT GROUPS. 



Probably the most conspicuous feature of the plant world to 

 the casual observer is the great number and diversity of forms. 

 It was formerly the custom to devote attention chiefly to the more 

 prominent groups of plants, or those that produce seeds, but more 

 recently the results of the studies on the less prominent groups, 

 as ferns, mosses, etc., have modified our views and made it 

 imperative that the botanist have a general knowledge at least 

 of all the great groups of plants. 



The most general classification of plants is that which divides 

 them into three great groups, — namely, (i) Thallophytes (Thal- 

 lophyta), (2) Archegoniates (Archegoniatae), and (3) Spermo- 

 phytes (Spermophyta). 



